I forgot about that. That is an excellent idea, especially if his tires are loaded with Rimguard. That is slimy and if you don't have enough air in the tires the wheels can spin in the tire. I guess we have established that nothing "bogs down", in other words the engine RPM does not decrease to the point of stall. TO the OP Words have meaning. Use more of them to describe your situation and be precise as you can. This will help us understand what may be wrong. So if the engine does not stall, and you seem pretty sure the hydrostatic transmission relief valve is NOT opening, then it must be something else. And the wheels spinning in the tires is certainly a possibility. Please also if you do find and solve the issue, come back here and let us know. Thank you in advance.