Hmm.

Check the elevator and make sure its not flexing between the two elevator halves. If both the elevator halves arent even with each other it will cause a roll.

Does the plane exhibit this behavior when you yank it up into a wall? Try pushing up into a wall inverted and see if it rolls.

This happens especially on planes with only one half of the elevator connected to the servo. In the larger planes you can correct it by running two elevator servos. In the foamies it can be easily remedied by putting a carbon fiber reinforcement between the two elevator halves so that they both travel the same amount.