303 hitec servos
I had the same problum and it turned out to be the servos.The hitec 300,303 and the futaba 3003 servos are know not to last very long and if not insalled right,viberation will kill the pots and make them act the way you are talking about.Change out the main servos in the like the elevator and the aileron servos to better ones like the futaba 148,hitec 422,airtronics 94102 or anything better.They are all around $14.00,I had A Global skylane and on the number three flite the servos was rubing on the rails and it went full to ful power and went stright in the runway becouse of the pots went bad.The radio was new also.I hope this helps-Mac