It could be something else in your radio, but it sounds like it could be one of your aileron servos has poor centering. It may just be a low end servo has started drifting or the pot has gone bad.

I test fly a lot of planes and poor centering servos are common. Some servos are more prone to this than others. Disconnect the aileron and tape a 6 inch long balsa stick to the servo arm. Tape a piece of paper to the wing under the servo. Move the aileron stick one way, release and mark where the end of the stick centers. Do this the other way and compare the 2 marks. If they are off much, replace the servo or put it on throttle. Check all your servos and it is best to put your most accurate servos on ailerons.