If your trim is acting up after having used an autopilot mode (such as diving forward excessively), try toggling on the level autopilot for a little bit. That, on my computer and with my controls, will cause the trim to revert to normal. It's almost as if the trim gets stuck in the trimmed autopilot position. Not sure this'll help you (I was on last night and we couldn't figure it out), but it's a thought.