Maybe the Tx is bad??? You don't have another laying around by chance, as you can swap the crystal to match the RX frequency and perahps narrow the true source of the problem. I love being able to swap out parts to diagnose a problem, only catch is you need two of everything.

Have you opened up the transmitter yet and looked for a loose wire or bad solder joint on one of the potentiometers (round do-dad with 3 wires connected to them) that move when the joystick moves??? Lots of flex sometimes on those wires when the sticks go back and forth.

Keep looking for the obvious stuff, as most circuits are very rugged and the problem is almost always something non-electronic part related.