I feel like I'm going through a sluggish slump with riding. Looking over my riding log I'm not over-training, I take enough rest days. I vary the intensity of my rides with some being strenuous and hilly and others being easy rides on flat trails. About 5 weeks ago I was feeling at the top of my form, today I felt like a slug. My legs just feel heavy and won't perform like I want them to and they poop out. In the last couple of weeks I start off very weak and about 1/4 to 1/2 way into my ride something clicks and I get a big burst of strength and stamina, often finishing quite strong and feeling like I could go longer. Today I had one burst of energy about 1/2 way into my ride but it didn't last long, just 2-3 miles and petered out. I finished, of course, but it was just so-so. Does this happen to you?