My 1975 ford 2000 raises the bush hog up to max height as soon as the engine is started. This crunches the drive shaft into the mower housing and will melt metal if the shaft is turning. If I engage and disengage the lever for draft control 3-4 times the 3 point hitch will operate properly again. When the tractor cools down the problem recurs. What is clogging up in the hydraulic system which makes this happen and how can it be remedied?