If I didn't know any better, it sounds like a vapor lock issue, which is really odd, especially in the winter. It always concerns me when a knowledgeable person like you has problems with trouble shooting. I'm sure you'll figure it out, and likely it'll be something so silly as to be stupid. Passing thought, have you checked the pick up tube for a burr? My buddy Scotty and I had a similar issue with a parts box build we did recently, the tube had been tightened with pliers which had obviously slipped. The float would catch at the top instead of sinking.