I would plug the servos into the receiver as you were using.
Turn the battery on and move the servos, wiggle wire, and tap on the control arms of the servos. Hopefully this will show a jitter or servo stoppage.
My gut feeling is that you have a servo problem. It would be very rare to have a transmitter issue, but ...
If you don't find anything then mark the servo and use it for a less critical function such as throttle.
Good luck!
Scott Hartman