first thing to do is swap the throttle and brake servo channels to see if the problem follows the servo to the other channel. that will tell you eally quick if its the servo.

Now if the glitch stays on the same channel even when you swap servos its a TX/RX problem. at that point try your TX on someone elses truck by switching to their channel on purpose (Duh ask them if you can test it of course!! lol) . if the glitch appears on the other persons truck its the Tx that has a problem. Lastly swap out the RX.