Since the wheel makes exactly one revolution, it almost has to be a bull gear problem. I thought about ice, but if you had it turning, it ought to crush any ice floating around, or it would fall off the bull gear. I'm going to go out on a limb and say you have a broken tooth on the bull gear.

The transmission cover is notorious for cracking and letting water in on those tractors. Try brazing up the corners where it cracks, that usually works. I have my brother TIG weld them, but it also helps that he is a welding engineer. Also, make sure the shifter boot is in good shape.