Two things, first make sure your ailerons are parallel to the airplane's thrust line at neutral, as "drooping" ailerons can cause some wierd trim problems, especially if combined with slight tail heaviness. Also closely check your aileron servo to make sure it returns to center. I had an aileron servo that would go several degrees "over center" every time I let off the stick, and I fought the trim for several flights and couldn't get it right. After changing the servo, all was well.

Also, make sure your engine doesn't have any upthrust in it, this can also give you some funky charicteristics and make trimming a nightmare.