Ok - looks like I spoke too soon.  On shutting down after running all day with no problems (largely unattended) the laptop crashed hard after trying to shutdown.

I found this in syslog after rebooting -- this is Linux 4.0:

Apr  2 20:30:07 monster kernel: [   69.841098] ACPI Warning: \_SB_.PCI0.PEG0.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150204/nsarguments-95)
Apr  2 20:30:07 monster kernel: [   69.841423] ACPI: \_SB_.PCI0.PEG0.PEGP: failed to evaluate _DSM
Apr  2 20:30:07 monster kernel: [   69.841428] ACPI Warning: \_SB_.PCI0.PEG0.PEGP._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150204/nsarguments-95)
Apr  2 20:30:22 monster kernel: [   84.853086] nouveau E[     DRM] failed to idle channel 0xcccc0001 [DRM]
Apr  2 20:30:22 monster kernel: [   84.853284] ------------[ cut here ]------------
Apr  2 20:30:22 monster kernel: [   84.853290] WARNING: CPU: 4 PID: 206 at /home/kernel/COD/linux/drivers/pci/pci.c:1546 pci_disable_device+0xab/0xc0()
Apr  2 20:30:22 monster kernel: [   84.853292] nouveau 0000:01:00.0: disabling already-disabled device
Apr  2 20:30:22 monster kernel: [   84.853293] Modules linked in: dm_crypt bnep rfcomm binfmt_misc asus_nb_wmi uvcvideo asus_wmi sparse_keymap snd_hda_codec_hdmi videobuf2_vmalloc videobuf2_memops videobuf2_core intel_rapl v4l2_common iosf_mbi snd_hda_codec_realtek x86_pkg_temp_thermal videodev intel_powerclamp snd_hda_codec_generic coretemp btusb media bluetooth kvm_intel snd_hda_intel snd_hda_controller kvm snd_hda_codec snd_hwdep snd_pcm crct10dif_pclmul crc32_pclmul snd_seq_midi snd_seq_midi_event ghash_clmulni_intel snd_rawmidi aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd snd_seq snd_seq_device snd_timer joydev serio_raw snd ie31200_edac mei_me mei shpchp soundcore lpc_ich edac_core mac_hid parport_pc ppdev lp nls_iso8859_1 parport nouveau i915 mxm_wmi ttm i2c_algo_bit drm_kms_helper drm psmouse ahci alx libahci mdio wmi video
Apr  2 20:30:22 monster kernel: [   84.853336] CPU: 4 PID: 206 Comm: kworker/4:1 Not tainted 4.0.0-040000rc6-generic #201503291935
Apr  2 20:30:22 monster kernel: [   84.853337] Hardware name: ASUSTeK COMPUTER INC. G750JS/G750JS, BIOS G750JS.208 07/17/2014
Apr  2 20:30:22 monster kernel: [   84.853341] Workqueue: pm pm_runtime_work
Apr  2 20:30:22 monster kernel: [   84.853342]  000000000000060a ffff880465403b88 ffffffff817e3106 0000000000000007
Apr  2 20:30:22 monster kernel: [   84.853345]  ffff880465403bd8 ffff880465403bc8 ffffffff810791b7 ffff880465403be8
Apr  2 20:30:22 monster kernel: [   84.853346]  ffff88046c366000 ffff88046c366000 ffff880035cd8800 00000000fffffff0
Apr  2 20:30:22 monster kernel: [   84.853348] Call Trace:
Apr  2 20:30:22 monster kernel: [   84.853353]  [<ffffffff817e3106>] dump_stack+0x45/0x57
Apr  2 20:30:22 monster kernel: [   84.853357]  [<ffffffff810791b7>] warn_slowpath_common+0x97/0xe0
Apr  2 20:30:22 monster kernel: [   84.853359]  [<ffffffff810792b6>] warn_slowpath_fmt+0x46/0x50
Apr  2 20:30:22 monster kernel: [   84.853363]  [<ffffffff8140b5c0>] ? pci_save_vc_state+0x40/0x100
Apr  2 20:30:22 monster kernel: [   84.853368]  [<ffffffff8140180b>] pci_disable_device+0xab/0xc0
Apr  2 20:30:22 monster kernel: [   84.853399]  [<ffffffffc03d8942>] nouveau_pmops_runtime_suspend+0x82/0x110 [nouveau]
Apr  2 20:30:22 monster kernel: [   84.853404]  [<ffffffff8140369a>] pci_pm_runtime_suspend+0x7a/0x160
Apr  2 20:30:22 monster kernel: [   84.853406]  [<ffffffff81403620>] ? pci_legacy_suspend_late+0x100/0x100
Apr  2 20:30:22 monster kernel: [   84.853408]  [<ffffffff81519fbd>] __rpm_callback+0x3d/0xc0
Apr  2 20:30:22 monster kernel: [   84.853411]  [<ffffffff810b1a1f>] ? dequeue_entity+0x17f/0x440
Apr  2 20:30:22 monster kernel: [   84.853413]  [<ffffffff8151a06f>] rpm_callback+0x2f/0x90
Apr  2 20:30:22 monster kernel: [   84.853415]  [<ffffffff8151a847>] rpm_suspend+0x107/0x5d0
Apr  2 20:30:22 monster kernel: [   84.853419]  [<ffffffff8101458e>] ? __switch_to+0xbe/0x5b0
Apr  2 20:30:22 monster kernel: [   84.853421]  [<ffffffff810b2f4f>] ? put_prev_task_fair+0x2f/0x50
Apr  2 20:30:22 monster kernel: [   84.853423]  [<ffffffff8151b66a>] pm_runtime_work+0xea/0xf0
Apr  2 20:30:22 monster kernel: [   84.853425]  [<ffffffff810920d4>] process_one_work+0x144/0x490
Apr  2 20:30:22 monster kernel: [   84.853427]  [<ffffffff81092b3e>] worker_thread+0x11e/0x450
Apr  2 20:30:22 monster kernel: [   84.853429]  [<ffffffff81092a20>] ? create_worker+0x1f0/0x1f0
Apr  2 20:30:22 monster kernel: [   84.853432]  [<ffffffff81098869>] kthread+0xc9/0xe0
Apr  2 20:30:22 monster kernel: [   84.853434]  [<ffffffff810987a0>] ? flush_kthread_worker+0x90/0x90
Apr  2 20:30:22 monster kernel: [   84.853437]  [<ffffffff817ef858>] ret_from_fork+0x58/0x90
Apr  2 20:30:22 monster kernel: [   84.853440]  [<ffffffff810987a0>] ? flush_kthread_worker+0x90/0x90
Apr  2 20:30:22 monster kernel: [   84.853441] ---[ end trace cb476b19202523ea ]---
Apr  2 20:30:22 monster kernel: [   84.853466] pci_pm_runtime_suspend(): nouveau_pmops_runtime_suspend+0x0/0x110 [nouveau] returns -16