First, check that when you have the seat belt locked-in, that the belt light indicator on the dash is in fact OFF. If not, there is an problem there. Next, when it does this stop leaving it run, then loosen the gas cap. If it sucks air in the tank check us stuck shut and the pump just can't deliver enough fuel pulling against the vacuum. Next, make sure it isn't in diff-loc. That will also limit the RPMs as well as a short on the reverse light indicator..make sure it also isn't lit. Double check that the hand brake is disengaging as well. Also might check that none of the wheel brakes are are locking up. Also with that many miles it will be time to check the valves so do that too.