Have a 2005 Sears LT2000 Lawn Tractor 18.5 HP. automatic transmission. Goes OK in reverse, but not in forward. Like it is slipping and catching and but just not wanting to go forward much. If you are on an incline or try to turn on the mower deck, forget it. Had it checked and they said no belt problems but they don't know much about transmissions. New mower that really hasn't had a whole lot of wear and of course right after the warranty period.

When put in gear and let brake off tractor stalls and when I engage blade it stalls

Sounds like a transmission problem. 6 speed? Do you leave it parked outside? If so you might have water in your xmsn with corrosion problems. Only way to tell is to take it apart and look.