This Cat T40D sometimes will not engage in forward or reverse. Had it transported to my facility on a Roll-back and went to attempt to drive it off the bed, it would not go under its own power, as it rolled down the ramp it suddenly engaged in forward & moved until parked. When restarted again & put in gear it would not engage in either forward or reverse. On occasion it will engage & drive normally in both forward & reverse with no slippage. Any time it successfully engages it drives normally with no slippage. Would really appreciate some idea's on how to determine the sollution.