Don't blame the cards on what you are tying to do. AMD and nVidia drivers do not work properly together.
HUGE ppd loss on r9 290 with a second GPU installed?! - Page 2
maybe something corrupted from using 2 different gpu driver? i would do a fresh OS installation and then only use cards from either amd or nvidia.
Why must AMD make everything so difficult...WHY.
Also I have ran AMD and NV in same rig before, I have been folding on my gtx 670 whilst gaming on my r9 290x no problems whatsoever. Seem to be really struggling this time.
If it worked before the card is fine. There are issues from something you have done and or drivers. Don't blame the card if it worked in the past, its not old and broke.
It's broken forever.
And on a second note, I've had no problem folding both Nvidia and AMD cards in the same system together before with optimal ppd.