Hip-

I have ridden both Conti GP 3000 tires and Mich Pro Race 2's and while I like the Pro Race 2's more, the difference in tires cannot explain the magnitude of the drop in performance that you are seeing. In fact, I have just put a used Conti GP 3000 on the rear of my road bike as a stop gap because I've worn out a Pro Race 2 and I've had one of my strongest weeks of the year riding.

Are you still monitoring your resting heart rate? If it is elevated that is a sure sign of overtraining.

I'm going to make a wild-ass guess here. Are you stressing out over this century ride coming up? Is your subconscience manufacturing an excuse not to try?