Q: Transmission problem

Of course, be sure the transmission fluid level is correct, the fluid is not contaminated and the filter is not clogged. A fault like the one you are describing is likely to be due to a hydraulic control issue. If the valve body leaks and/or there are faults in any of the o-rings or piston seals (those are rubber parts and subject to eventual failure) the transmission can fall out of gear. It’s also possible that a solenoid is cutting out although your description of this occurring under load points more toward a hydraulic fault. Although the problem can be diagnosed on the vehicle, using for example, external pressure gauges, and "possibly" be repaired on the car, if the transmission truly has 321,000 the most efficient strategy is to refurbish (rebuild) the whole unit because it probably has lots of wear and all seals should be replaced anyway. If you desire a diagnostic and a precise identification of the issue, please simply request a shift diagnostic and the responding certified mechanic will get this resolved for you. Let us know if you have additional questions or concerns.