Could it be your brakes? I've read a couple of posts where there are problems with the brakes 'freezing' (e-brake especially) in cold weather. Could be that when you drive (you're hitting the accelerator and may not feel the additional drag) and then throw it into neutral (and let off the gas), the brakes are dragging your speed back down??? Would make sense, too, since the brakes would be warming up about like the engine does, so you would notice the drag lessening with the engine warming.... Dunna, just a shot in the dark.