Here's how to tell if a free wheel or not-
http://sheldonbrown.com/free-k7.html
I'd just assumed it was a free wheel because it's a Schwinn.
I actually looked at Bikepedia and it appears at least some of them were 8 speed.
MOST 8 speeds are cassettes, but 8 speed free wheels do exist.
As a 2nd thought, I see you are in Denver. Is it cold enough that any internal moisture inside is freezing?Probaly not an issue for most bikes, but possibly if the bike is left outside where it can get rained on.