Problem with Drive Train
 

My fiancee has a Giant Rincon SE and uses it as an all-weather commuter/utility/long-distance bike.
During the past couple of winters, especially during heavy cold winds she's experienced problems with the drive-train. Sometimes the tension in the system goes out and she is able to freely pedal with no resistance from the chain. The problem is not a skipping chain. This happens for at least 5 seconds at a time, several times within a mile.
My amateur mechanic mind thinks the problem might be the freehub's rachet-arms are getting stuck/frozen in the sub-zero conditions, thus not moving the wheel when the cassette turns the freehub. Do your expert minds agree with this theory or could there me other causes?
Thanks!

Yes, thick lubes get thicker when cold, and also stickier. the pawls have very light springs so it's easy for them to get glued in the retracted postition by sticky oil or grease. Also grease can fill the base of the ramps causing the pawls to ski above the faces they need to engage.

Winter's over, so do your normal service, especially if they use salt on the roads where you've been riding. Continue to use the same lube that served well in the summer, but next fall flush the freehub and relube with a lighter synthetic oil. (Synthetics don't change viscosity as much with temperature change as non-synthetics do)