I am facing the same thing. Mon-thurs is recovery until spring, fri-sun are riding days, with fri being my freshest fastest day.

One fri I killed my ride, felt great and wasn't sore, just tired. The next day on a group ride, at the very beginning we start with a small climb and I had nothing, no power at all. Had to tell the rest of the group to go on that I was gonna be slow that day.

My fresh day speed is around 16 mph. Dead leg days are 2/3 that speed. They suck!

Currently looking at everything from nutrition before, during and after to figure out if its caloric, glycogen levels or just plain old muscle fatigue. I've only been cycling since last sept and was never athletic before, so a rank old newb.