The above posts probibly are right.

However, I've seen this effect in some of my combat planes, exaclty like you describe. It's usually a tail heavy condition. You are basically every so slightly unstable. Move the CG forward 1/8" - 1/4" should do it. I've also fixed the problem by raising the ailerons (reflex) a turn or three on the clevis. I don't know why this works (maybe changing the pitching moment?), but I do know that it works in some cases.