This is my first half-season with a 2007 LA165 24up Briggs and Stratton 48 inch mower deck that I bought used with about 300 hours on it from a local John Deere dealer. I mow about a 1/3 of an acre that is relatively flat but does have a short 45 degree slope at the back of the yard. Now I'm not a small guy at 6'3 and about 380lbs. What I'm experiencing is that after about a half hour of mowing the mower slows down until it stops moving. I do not notice any problems with the engine and the mower deck. This problem has happened once before and the problem went away for a bit after I changed the belts. With a hydrostatic transmission I keep my foot on the forward pedal and I do not use the cruise. I was angry when it happened so I just waited for the mower to cool off a bit and then parked it in the garage and walked away. I'm somewhat handy so when I get around to fixing this thing what are some of the causes that I should be looking for? Any help would be most appreciated. Thanks.