I had this same problem with a rookie and it took me a long time to find. Are the ailerons live hinged? If so, check the skin where the hinge is. Check to make sure that skin is still securely glued to the trailing edge spar. I found on my rookie that the glue joint between the skin and the balsa rear spar had come apart and caused this exact problem. The faster the airplane went, the worse the roll got.