Perhaps low idle.
Does it "stall" like the motor is overloaded, or does it just shut off, like someone turned the key. If the later, is a safety interlock not satisfied or reading a false positive?
That is, what are the normal safety interlocks? Is there a condition where tractor shuts off if PTO is on and something else occurs ( Example: If nobody is in seat, or if you put it in reverse, etc...)? If so, it might be those switches that aren't working correctly.