I'm not sure now. I restarted the PC and now the core clock is back up to 1500Mhz (it was like 405Mhz or something in the P1 state) but now that it's at 1500Mhz I am only getting 25k PPD on a Core18 WU. Restarted the client and, while it took a while it is now reading ~325K PPD.
It's the top GPU in the the rig, so I'm wondering it maybe it overheated? Earlier I noticed the temp was up in the mid to uppers 70s and the fan was set to auto. I had previously set both fans to 100% 24/7 and somehow it changed just that one GPU to auto while the other GPU (the lower one) was running 100% fan speed and around 47C.
That sounds like a Nvidia driver crash and it does require a reboot to fix it, you may want to lower the OC by 5MHz or 10MHz if it happens again.