I've read through this thread a couple of times now and had nothing to offer. Then it dawned on me that this sounds like a control surface centering problem. I think Vertical Grimmace is right. But it isn't necessary the fault of the servo. Even a high dollar, high torque, super accurate servo can have centering problems if the linkage isn't right.
If centering isn't dead on accurate it will cause exactly what you describe, this is frequently caused by stiff linkage or hinging. Remove the pushrod clevis from the horn and make sure the surfaces are very free to move. No glue in the hinges, no binding torque rods, no off-axis hinges. Lots of things can lead to binding. Also check the linkages. Is everything free? Quality servo and good centering?
How are the ailerons set up? Torque rods? 1 or 2 servos? Servos with direct pushrods to surface? All of these answers change the equation. What kind of hinges? Details help the diagnosis.
My bet is on a little bit of control surface binding that is causing centering problems. Especially since you say it rolls one way, you correct for it (moving the control surface the other way) and then it rolls the other way.
Take a look at the ailerons and see if they always returns back to the exact same center position if you move the stick to the left or right. But return the stick to the center position very slowly in both cases. If you see any difference in aileron position when you return to center , you've got binding and the result will be exactly what you described. Centering is so critical that sometimes the centering looks good enough to the eye but it's not accurate enough for flight conditions. So just because it looks right, doesn't mean it is. Do you feel any friction at all in the hinging?
Dave
Last edited by dbacque; 04-28-2015 at 03:53 PM.