Right, had time to do a couple tests on the Feb build:
First quick render on the W7/GTX980 machine, renders no prob. Tried lowering the samples as per your 20% comment, can’t see any difference in my results - looking close but not microscopic inspection!
1280x720, 100%, 300 samples = 8 mins
1280x720, 100%, 250 samples = 6m 35secs
As for the Win10/GTX970 machine - it works fine! Don’t know what caused the previous issues, but this time i F12 w/ the full settings (1280x720 at 100%, 300 samples) and it just tore through it in under 7 mins (sorry, didn’t time it, was just please it was working). Thought it was playing w/ me at first, got right to the end and closed Blender, but then i noticed Windows was doing some update thing so maybe that messed w/ it, the next time i tried a render, all was good.
Edit - just for clarity, my 970 isn’t overclocked so i obviously had a different issue to the above.