Put another servo on that channel and see if it does the same thing. I'd lean more toward a bad RX--when the ailerons were Y'd together, were they on Channel one? I'm not that familiar with futaba, but I think ailerons is channel one, aren't they? Then both servos were sticking at full throw when hooked to channel one, and only the one hooked to channel one stuck when they were separated? Something is wrong with channel one--either with the RX, or TX. FIx it before you fly, or it will cause a crash...