I have had similar experiences while running and swimming (for both I have trained for racing and long distance). For me, it is a signal that I need to add sprints to my workouts . . . or incresase the length of my sprints if I am already doing them. . . where you push hard for two minutes, rest 30 seconds, etc. I find these are better on workouts alone and I add these every other day. I am new to biking, but I imagine the theory is the same. Nutrition, fuel, strength training, stretching and sleep are also important. Lots of interesting research is out there with endurance sports.

Sandy