Olde, I was reading your posts and was wondering if you have checked the gear oil for your tractor. Depending on the oil in the rear end it could be part of your problem. It might be time to service it if you havent already. There are about 3 or 4 drains to help remove the gear oil. The filter/screen is attached to the bolt that holds the bottom line coming from the hydro pump to the transmission. If somebody changed the gear oil to a thicker 85w-140 to boost line pressure the pump could be having trouble pumping it in the cold weather. The screen could be full of trash as well. Might be a good idea to change the oil and clean the screen just to be safe. I have had better luck with the full synthetic gear oils during the winter flowing easier than the standard gear oil.