By "tab out" does it just drop to the desktop or is it actually "black screening" for a few seconds?

The display driver crashing doesn't always display the windows message for whatever reason, the exact problem you're describing happens to me in a few other games (black screen, recovery, terrible FPS).

The only way I've found to fix this is to alt+tab out, wait a few seconds, and then alt+tab back in. It seems to fix the low fps about 90% of the time.