With an 8 speed cassette you have a freehub, not a freewheel.
In either case I would remove the cluster and address what is more than likely some water contamination and it needs to be flushed a re-lubed, preferably with a synthetic marine grease which will resist water and will handle the coldest temperatures you can imagine.
With freewheels I have a snappy little grease injector that threads into the back and with a grease gun I can reload the grease, after flushing it out of course.
Freehubs are easier to service if you have the right tools... the Park website has an excellent how to on this and will save me a bunch of typing.
http://www.parktool.com/repair/readhowto.asp?id=45