Another point I keep forgetting to clarify is your nVidia Inspector settings. Are you running that 1/2 frame limit, for 30 fps, and / or the "Vertical Sync" check-boxed? I had forgotten to check those settings on my install, so now I've got one with the nVidia Inspector set with 1/2 frame, and the other with Vertical Sync set - but I think that computer (the Win10 laptop) just died... They aren't playing taps just yet, but it is DOA on the surgery table right now...
- of course, it has done this before...
shtinking Windows updates