My toro WH runs great for about 5 min.s then slows to a stop , I replaced the belt and double checked the adjustments so its running tight but after warming up slows to a crawl , if I disengage the blade i can get it back to the shop . after cooling it will go agin just to do the same . when it stops it will move very slow in reverse only (until) I disengage the blades, any thoughts?? PS this is a hydostactic drive