I also experience this problem. I've been having it for a while. Logging out and logging in again temporarily fixes the issue. Affects both KDE and Gnome. Got this in /var/log/syslog when the problem happened just now:

Mar 31 21:57:54 dreadnought kernel: [ 2390.058249] irq 16: nobody cared (try booting with the "irqpoll" option)
Mar 31 21:57:54 dreadnought kernel: [ 2390.058253] CPU: 0 PID: 0 Comm: swapper/0 Tainted: PF          O 3.11.0-18-generic #32-Ubuntu
Mar 31 21:57:54 dreadnought kernel: [ 2390.058254] Hardware name: System manufacturer System Product Name/P8P67-M, BIOS 1501 08/30/2011
Mar 31 21:57:54 dreadnought kernel: [ 2390.058255]  ffff880400974c9c ffff880417403e68 ffffffff816e8571 ffff880400974c00
Mar 31 21:57:54 dreadnought kernel: [ 2390.058257]  ffff880417403e90 ffffffff810f88e2 ffff880400974c00 0000000000000010
Mar 31 21:57:54 dreadnought kernel: [ 2390.058259]  0000000000000000 ffff880417403ed0 ffffffff810f8d11 ffffffff815a158f
Mar 31 21:57:54 dreadnought kernel: [ 2390.058261] Call Trace:
Mar 31 21:57:54 dreadnought kernel: [ 2390.058262]  <IRQ>  [<ffffffff816e8571>] dump_stack+0x45/0x56
Mar 31 21:57:54 dreadnought kernel: [ 2390.058270]  [<ffffffff810f88e2>] __report_bad_irq+0x32/0xd0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058271]  [<ffffffff810f8d11>] note_interrupt+0x141/0x200
Mar 31 21:57:54 dreadnought kernel: [ 2390.058274]  [<ffffffff815a158f>] ? cpuidle_enter_state+0x4f/0xc0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058276]  [<ffffffff810f6681>] handle_irq_event_percpu+0xe1/0x1e0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058277]  [<ffffffff810f67bf>] handle_irq_event+0x3f/0x60
Mar 31 21:57:54 dreadnought kernel: [ 2390.058279]  [<ffffffff810f988a>] handle_fasteoi_irq+0x5a/0x100
Mar 31 21:57:54 dreadnought kernel: [ 2390.058283]  [<ffffffff81014b8e>] handle_irq+0x1e/0x30
Mar 31 21:57:54 dreadnought kernel: [ 2390.058285]  [<ffffffff816fa70d>] do_IRQ+0x4d/0xc0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058287]  [<ffffffff816efd6d>] common_interrupt+0x6d/0x6d
Mar 31 21:57:54 dreadnought kernel: [ 2390.058288]  <EOI>  [<ffffffff815a158f>] ? cpuidle_enter_state+0x4f/0xc0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058291]  [<ffffffff815a1588>] ? cpuidle_enter_state+0x48/0xc0
Mar 31 21:57:54 dreadnought kernel: [ 2390.058293]  [<ffffffff815a16c9>] cpuidle_idle_call+0xc9/0x210
Mar 31 21:57:54 dreadnought kernel: [ 2390.058295]  [<ffffffff8101bafe>] arch_cpu_idle+0xe/0x30
Mar 31 21:57:54 dreadnought kernel: [ 2390.058298]  [<ffffffff810b5825>] cpu_startup_entry+0xe5/0x280
Mar 31 21:57:54 dreadnought kernel: [ 2390.058300]  [<ffffffff816d7767>] rest_init+0x77/0x80
Mar 31 21:57:54 dreadnought kernel: [ 2390.058303]  [<ffffffff81d26ef7>] start_kernel+0x40a/0x416
Mar 31 21:57:54 dreadnought kernel: [ 2390.058305]  [<ffffffff81d268f6>] ? repair_env_string+0x5c/0x5c
Mar 31 21:57:54 dreadnought kernel: [ 2390.058307]  [<ffffffff81d26120>] ? early_idt_handlers+0x120/0x120
Mar 31 21:57:54 dreadnought kernel: [ 2390.058309]  [<ffffffff81d265de>] x86_64_start_reservations+0x2a/0x2c
Mar 31 21:57:54 dreadnought kernel: [ 2390.058310]  [<ffffffff81d266e8>] x86_64_start_kernel+0x108/0x117
Mar 31 21:57:54 dreadnought kernel: [ 2390.058311] handlers:
Mar 31 21:57:54 dreadnought kernel: [ 2390.058323] [<ffffffffa0064490>] e1000_intr [e1000]
Mar 31 21:57:54 dreadnought kernel: [ 2390.058373] [<ffffffffa0797a70>] nv_kern_isr [nvidia]
Mar 31 21:57:54 dreadnought kernel: [ 2390.058374] Disabling IRQ #16

willz@dreadnought:/proc$ cat interrupts | grep 16:
 16:     600008          0          0          0   IO-APIC-fasteoi   eth1, nvidia

willz@dreadnought:/var/log$ uname -a
Linux dreadnought 3.11.0-18-generic #32-Ubuntu SMP Tue Feb 18 21:11:14 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

willz@dreadnought:/var/log$ nvidia-settings -v
/usr/share/themes/Blumix1.0/gtk-2.0/gtkrc:580: Unable to locate image file in pixmap_path: "transparent_dot.png"
/usr/share/themes/Blumix1.0/gtk-2.0/gtkrc:583: Background image options specified without filename

nvidia-settings:  version 304.88  (buildd@panlong)  Thu Jul 11 10:40:14 UTC 2013
  The NVIDIA X Server Settings tool.

  This program is used to configure the NVIDIA Linux graphics driver.
  For more detail, please see the nvidia-settings(1) man page.

  Copyright (C) 2004 - 2010 NVIDIA Corporation.

Let me know if you want more information. This issue bugs me a lot.