It could be any # of things that are causing your airplane to re-trim so drastically. First and foremost, I would do what you need to do to lose the elevator trim. Any big "trim" adjustments at the control surfaces may cause trim changes at different speeds.

While most say to put the wing and engine and "zero", and adjust the tail with a bit of negative incidence, I would put the engine and tail at "zero", with some positive in the wing. Here is why. The only thing the tail really does is adjust the angle of attack of the wing. "Up" trim in the tail is pushing down on the tail, to increase the angle of attack on the wing. This scenario actually puts the tail at or closer to "zero", but the wing AND engine with some positive. It also puts the whole fuse in a positive angle of attack. In my opinion, this is more drag than simply having the wing set with some positive incidence, with everything else at zero.

Correct it however you're most comfortable, but you should be better off with the airplane adjusted so that it carries very little trim.