OS/CPU/RAM/GPU/vRAM, the number-dot-number version of PrPro you're using, and the media involved would all be useful.

Next ... what is Mercury Acceleration set to in the Project settings dialog?

And have you checked that your GPU's driver is the latest available? Often times, the GPU drivers aren't always updated ... and sometimes the 'latest' update causes issues and one needs to roll back a driver.

Neil