I'm having similar problem with my Craftsman, and I just discovered what I believe is the problem. There is a round-ish plastic plate/cover that is mounted inboard of the front wheels that is designed to keep clippings and debris out of the area where the drive gear meshes with the teeth on the inside of the wheel. This plastic cover and the wheel itself each have a flange, and when the wheel and cover are in place, these flanges are located in very close proximity, and are fairly effective in blocking debris from getting up into the vicinity of the gears.
The problem is that when rolling over thick or even slightly uneven ground the plastic cover can drag the ground and this will force the cover's flange to bind against the flange of the wheel. Once these flanges become in a "bound" state they tend to remain engaged and since the cover does not turn, the wheel does not turn as well.
I believe this is a design flaw that might be resolved by either:
1. obtaining larger front wheels which would cause the plastic flange to be higher above the ground and thereby less likely to come in contact with the ground,
2. to re-engineer or modify the plastic cover, and/or its flange, or
3. to remove the plastic cover altogether.
I intend to look at the parts shops to see if there are replacement front wheels that will fit this mower, but assuming that is not achievable, I am going to trim the plastic cover's flange - e.g., removing the flange from the very bottom /lower portion of the cover. This is the portion of the flange where the cover and the wheel's flanges are binding. A larger wheel might be the optimum, but I am pretty confident that trimming a few inches of the plastic flange will greatly improve the situation.