It's possible you have an electrical problem that made it stop working entirely, as Brett suggested. But I think your original cold/hot problem is due to worn internal seals. As the oil warms up, it thins out and leaks past the seals more easily until the OD drops off from insufficient oil pressure.

All the "high milers" I know, like Irv Gordon, Art Banks, etc. use 80/90 gear oil in theirs, even though the manual specifies 30W. That might band-aid your problem for a while, but sounds like it needs a rebuild.