Well, that sucked. Got 2/3 to school and then I had the issue. This time, however, I was dead in the water. I was able to shift a couple times at fix the solution temporarily. Once I got a bit farther, however, I was done.
Basically, I still think there is a chain tension issue to some extent. There may also be some freezing going on... the rear derailleur would catch sometimes if you pedal backwards.
The biggest issue, however, is even if pedaling forward, the cassette will rotate with the chain and not move the wheel. Once in awhile it would catch momentarily, but then it would slip again permanently. I'm guessing this is is definitely a freewheel/hub problem - given the temperature dependence, I assume moisture/lube in there is freezing up and preventing it from catching?
So final question - how hard is it to clean this thing out? Do you need any special tools?