Thank you Eric. The computer crashed just now, at 14:41 my time. A few minutes before a fan turned on on or close to max. I've found the following in Console - All messages:
2015-10-04 14:41:43,000 kernel[0]: [1:0:0] GPU HangState 0x00000000, HangFlags 0x00000006: IndividualEngineHang 0, NonEngineBlockHang 1, FenceNotRetired 1, PerEngineReset 0, FullAsicReset 1
2015-10-04 14:41:43,000 kernel[0]: ** GPU ASIC Log Start **
2015-10-04 14:41:43,000 kernel[0]: : 0x00006898 : 0x00000092 : 0x00000001 : 0x00000018 : 0x000090f0 : 0x00000001 : 0x00000001 : 0x00006898
2015-10-04 14:41:43,000 kernel[0]: : 0x00000001 : 0x00000015 : 0x00000000 : 0x00000001 : 0x00000047 : 0x00000000 : 0x00000001 : 0x00000058
Similar entries continues for several pages, then this:
2015-10-04 14:41:44,000 kernel[0]: ** GPU ASIC Log End **
2015-10-04 14:41:44,000 kernel[0]: ------------------
2015-10-04 14:41:44,000 kernel[0]: GPURestartReportEnd
2015-10-04 14:41:44,000 kernel[0]: Trying restart GPU ...
2015-10-04 14:41:45,816 com.apple.launchd[1]: (com.apple.DumpGPURestart) Throttling respawn: Will start in 9 seconds
2015-10-04 14:41:46,000 kernel[0]: [1:0:0] GPU HangState 0x00000000, HangFlags 0x00000006: IndividualEngineHang 0, NonEngineBlockHang 1, FenceNotRetired 1, PerEngineReset 0, FullAsicReset 1
2015-10-04 14:41:46,000 kernel[0]: ** GPU ASIC Log Start **
2015-10-04 14:41:46,000 kernel[0]: : 0x00006898 : 0x00000092 : 0x00000001 : 0x00000018 : 0x000090f0 : 0x00000001 : 0x00000001 : 0x00006898
2015-10-04 14:41:46,000 kernel[0]: : 0x00000001 : 0x00000015 : 0x00000000 : 0x00000001 : 0x00000047 : 0x00000000 : 0x00000001 : 0x00000058
Which also continues for pages. Then:
2015-10-04 14:41:47,000 kernel[0]: ** GPU ASIC Log End **
And then another several pages of similar kernel[0] entries, then a similar set as above:
2015-10-04 14:41:47,000 kernel[0]: : ** GPU Debug Info End **
2015-10-04 14:41:47,000 kernel[0]: ------------------------
2015-10-04 14:41:47,000 kernel[0]: GPURestartReportEnd
2015-10-04 14:41:47,000 kernel[0]: Trying restart GPU ...
2015-10-04 14:41:47,019 com.apple.launchd[1]: (com.apple.DumpGPURestart) Throttling respawn: Will start in 8 seconds
2015-10-04 14:41:48,000 kernel[0]: [1:0:0] GPU HangState 0x00000000, HangFlags 0x00000006: IndividualEngineHang 0, NonEngineBlockHang 1, FenceNotRetired 1, PerEngineReset 0, FullAsicReset 1
2015-10-04 14:41:48,000 kernel[0]: ** GPU ASIC Log Start **
Under Console - System Diagnostic Reports I also see a crash report for the PenTabletDriver with the 14:41 time stamp. Oddly, this morning I removed the driver and software for the Wacom tablet and reinstalled it and it worked well all day.
Not sure what this means but it appears both the GPU and the tablet are upsetting the system somehow.