Mike, had this twice as well on my single PCIe slot Dell box, as soon as I took any AMD/ATI card out and put any of my GTX in instead, BUT, and that is the point, left the AMD/ATI drivers installed.
For some stupid reason, NVidia drivers refuse to work, when there is an AMD/ATI driver installed for the exact same slot. Competition BS from NVidia to hassle AMD owners, I guess.
From the messages output you were so wise to post I conclude there is no other graphics card in this system and from my recent poor experience with NVidia drivers regarding their competitor I conclude this must be it. Use DriverFusion (successor of DriverSweeper) to clean any AMD/ATI remnants directly after stock uninstall as well, then reboot!
After this procedure, my GTX 480/680 always purred along happily...
Hope this guessing in relative dark of mine helps!
Oh and reduce the number of parallel POEM tasks in your app_info to four, I suggest, since one needs 256 MB and you have 1024 MB; had the same with my GTX 480 (1536 MB) and seven instead of six, instantly the average runtime jumped a lot higher than sixteen percent relative to six. IIRC it was around 23 percent higher relatively! Reason as far as I could determine: the data with seven at once was copied out from RAM and into the VRAM and vice versa all the time, as the memory controller was double as active relative to six at once...
For the same reason watch the VRAM clocks! When the driver has reset the VRAM clock is at lowest possible stock and the average runtime leaps in an unwanted direction. At least that was the case twice with one of my GTX 680s I had kicked a little too hard too far.