Originally Posted by Myosmith

I think you hit the nail on the head. Slow decreases in performance in the setting of frequent hard riding is generally a sign of overtraining. Remember that other stresses in your life, especially those that interfere with sleep, relaxation, and nutrition will change your threshold for overtraining. Take a physical and mental break with lots of sleep, good nutrition, and some enjoyable active rest (pleasant, low intensity recreation) for a few days and then schedule rest periods throughout your training. You don't get stronger when you ride, you get stronger when you rest after you ride.