Hudgin, what you describe is the classic heavy wing. There is a bit somewhere on VANS website regarding how to fix it (see below), but let me explain briefly.
First if the ailerons are out of line in some way you want to fix that.
Before you investigate the ailerons though, make sure the ball is in the center, because if you are flying yawed, the ailerons will have differential lift.
Assuming that is all resolved and you still have the problem the issue works like this. With the stick in the middle the ailerons are all ligned up and she flies correctly, however if you let go she roles left or right. I forget which way for you, but whatever.
The issue is this. One aileron is developing more lift than the other. While this is so slight it does not roll the aircraft while you hold the stick, it is enough to lift an aileron if you let go. The aircraft will then role towards the lifting aileron and will continue to roll since it will remain up.
The common solution is then to squeeze the aileron of the non-heavy wing. This causes its aileron to produce less lift. Its complicated as to why but if you want to understand, and like maths, read up about the Kutta Condition.
I recently had all these problems and currently have a bit of wood to bias first the rudder and then half a coffee stirer stick to bias the aileron. I havnt done the squeeze yet. If you want to see pictures go
here. About half way down. You will find links to VANS article and an explanation of the Kutta condition.