Ok, I think I got it.

The glitches occur with or without the engine running, with or without the failsafe connected, and on channels 1 and 2 if I switch the wires on the receiver.

With that said, it's not radio interference, not a faulty failsafe, and not the reciever. It's the servo. It freaks out every time it gets power, and ignores all incoming signals, whether it be from a failsafe or a radio command.