Dan I have had the same problem for over a year and found the answer today. Mine is an 800 series. In front of the transaxle there is a metal bracket that is attached to both sides of the frame. In the bracket there are two holes that are supposed to have bolts in them attaching it to the transaxle to keep the axle from torquing under a load. Mine were missing. When the axle torques forward, the belt comes off. Since there are numerous unused holes, I had not picked up on the fact that there were missing bolts. I talked to the local MTD shop and they told me what the problem was without even seeing the tractor. I ran two self tappers into the holes and the problem is solved. While you are at it, drill out the self tapping screws that hold the bracket to the frame and put in bolts and nuts. In fact those self tappers work for the axle to bracket fix.
Roy, September 2004
