Severian,

I have this problem where I think I can go out and ride as fast as when it's warm but my system just can't be pushed as much in the cold weather. Eating could be part of it. But I think it mostly has to do with the fact that I usually drop down to 2-3 hours riding per week instead of 5 -8. And for some reason my system just is not as efficient in the cold. I really noticed this when the tempertures dropped below about 65 degrees F.

You might also try to dress just a touch warmer (not too much). I have found that if my body feels cold it will never performe well no matter how hard I work out.

It's also possible that you have caught a mild cold and it only shows in exertion fatique.