Hmm. What servo are you using? You know my buddy has an OMP Edge 540, and was using a Hitec and everytime he flew it the elevator trim would be off ONLY when he flew. On the ground you couldn't tell unless you checked everytime. His first flight resulted in loops! However, he swapped a new Hitec and all is good. I think the old one has had better days, being in a few planes before the profile.

Could very well be C of G, and sounds like it. When an elevator is sensitive, that tells me its tailheavy.