RE: previous comment - my cables are connected and screwed in tightly. I often switch the monitors off when leaving at night. 99% of the time the machine is left running and has no problems. Sometimes, however, like again today, the X-Server HANGs !! : $ gdb /usr/bin/Xorg 1855 GNU gdb (GDB) 7.9 Copyright (C) 2015 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-unknown-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: . Find the GDB manual and other documentation resources online at: . For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from /usr/bin/Xorg...(no debugging symbols found)...done. Attaching to program: /usr/bin/Xorg, process 1855 Reading symbols from /lib/x86_64-linux-gnu/libudev.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libselinux.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libgcrypt.so.11...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libdl.so.2...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libpciaccess.so.0...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libpthread.so.0...(no debugging symbols found)...done. [New LWP 2399] [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Reading symbols from /usr/lib/x86_64-linux-gnu/libdrm.so.2...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libpixman-1.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXfont.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXau.so.6...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxshmfence.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXdmcp.so.6...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libaudit.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libm.so.6...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libc.so.6...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libcgmanager.so.0...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnih.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnih-dbus.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libdbus-1.so.3...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/librt.so.1...(no debugging symbols found)...done. Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libpcre.so.3...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libgpg-error.so.0...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libz.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libfreetype.so.6...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libbz2.so.1.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libfontenc.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libpng12.so.0...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libgcc_s.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/xorg/modules/extensions/libglx.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libglapi.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXext.so.6...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXdamage.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXfixes.so.3...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libX11-xcb.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libX11.so.6...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb-glx.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb-dri2.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb-dri3.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb-present.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb-sync.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libxcb.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libXxf86vm.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/xorg/modules/drivers/radeon_drv.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libdrm_radeon.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libglamor.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/xorg/modules/libexa.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/xorg/modules/libfb.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libgallium.so.0...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libelf.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libexpat.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libLLVM-3.4.so.1...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libstdc++.so.6...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libffi.so.6...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libtinfo.so.5...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libtxc_dxtn.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/xorg/modules/input/evdev_drv.so...(no debugging symbols found)...done. Reading symbols from /usr/lib/x86_64-linux-gnu/libmtdev.so.1...(no debugging symbols found)...done. Reading symbols from /lib/libnss_centrifydc.so.2...done. Reading symbols from /lib/x86_64-linux-gnu/libresolv.so.2...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnsl.so.1...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnss_compat.so.2...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnss_nis.so.2...(no debugging symbols found)...done. Reading symbols from /lib/x86_64-linux-gnu/libnss_files.so.2...(no debugging symbols found)...done. 0x00007fe1ecc2a337 in ioctl () from /lib/x86_64-linux-gnu/libc.so.6 (gdb) t a a bt Thread 2 (Thread 0x7fe1e5324700 (LWP 2399)): #0 0x00007fe1ee126414 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib/x86_64-linux-gnu/libpthread.so.0 #1 0x00007fe1e80b84c3 in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so #2 0x00007fe1e80b7e07 in ?? () from /usr/lib/x86_64-linux-gnu/dri/r600_dri.so #3 0x00007fe1ee122182 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0 #4 0x00007fe1ecc3347d in clone () from /lib/x86_64-linux-gnu/libc.so.6 Thread 1 (Thread 0x7fe1ef4089c0 (LWP 1855)): #0 0x00007fe1ecc2a337 in ioctl () from /lib/x86_64-linux-gnu/libc.so.6 #1 0x00007fe1edf11804 in drmIoctl () from /usr/lib/x86_64-linux-gnu/libdrm.so.2 #2 0x00007fe1edf13e3f in drmCommandWriteRead () from /usr/lib/x86_64-linux-gnu/libdrm.so.2 #3 0x00007fe1e8a76357 in ?? () from /usr/lib/x86_64-linux-gnu/libdrm_radeon.so.1 #4 0x00007fe1e8cc5738 in ?? () from /usr/lib/xorg/modules/drivers/radeon_drv.so #5 0x00007fe1e8cc5ba8 in ?? () from /usr/lib/xorg/modules/drivers/radeon_drv.so #6 0x00007fe1ef079054 in _CallCallbacks () #7 0x00007fe1ef1caabb in FlushAllOutput () #8 0x00007fe1ef073db3 in ?? () #9 0x00007fe1ef077d9a in ?? () #10 0x00007fe1ecb5aec5 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6 #11 0x00007fe1ef0631ee in _start () (gdb) gdb) info reg rax 0xfffffffffffffe00 -512 rbx 0xffffffff 4294967295 rcx 0xffffffffffffffff -1 rdx 0x7fe1f05c1708 140608376936200 rsi 0xc0206466 3223348326 rdi 0xc 12 rbp 0x7fe1f05c1708 0x7fe1f05c1708 rsp 0x7ffeb9e0a218 0x7ffeb9e0a218 r8 0x7fe1f2c90130 140608417628464 r9 0x696 1686 r10 0x7fe1f2c90120 140608417628448 r11 0x246 582 r12 0xc0206466 3223348326 r13 0xc 12 r14 0x7fe1ef450858 140608358647896 r15 0x7fe1ef460990 140608358713744 rip 0x7fe1ecc2a337 0x7fe1ecc2a337 eflags 0x246 [ PF ZF IF ] cs 0x33 51 ss 0x2b 43 ds 0x0 0 es 0x0 0 fs 0x0 0 gs 0x0 0 There is a Radeon card specific ioctl being invoked here that sometimes causes the X-Server to lock up.