Late to the party, but just in case somoene else ends up needing some help with this problem and this thread doesnt help (like it didn't help me)... I wanted to share with you all the fact that my own GTX 970 was performing very strangely: tf2 would flit between ~120 and 20 fps in about 1 second, making the game unplayable. I resolved it by exiting steam and re-running as an administrator. Cheers!