Hi, Regiment
Suggest you start with the absolute basics
- are the receiver batteries working? Try a pack of four ordinary alkaline cells to give the Rx plenty of voltage
-try plugging the receiver into the battery without the transmitter on (NEVER do this normally!) are the servos twiching then?
if one servo twitches whatever the radio is doing (Transmitter on or off) then its probably a servo with faulty/dirty/wet potentiometer. Check by substituting another servo. If this works fine you had a servo problem
Or try swapping the servo plugs in the receiver and see if that swaps the jitter over
Servos with faulty/dirty/wet pots can be repaired/fixed but with the cost of servos where it is now not many people would bother to do that?
The basic thing you are seeing is that the servos don't or can't obey the radio position signal. if both servos are twitching or moving then the radio is (probably) the problem - check batteries in both transmitter and receiver. If only one servo is doing it try changing them over as above or try another good, dry servo
hope this helps a bit
andrew