[Chrislong]

Go through all these things to eliminate the issue...

Change Crystal

Check receiver aerial length

Clean out the receiver, making sure the receiver plugs make clean contact

How old are your tranny batteries? replace them (could have 1 cell down)

Is your crystal near carbon, ifso, stand the receiver on its side - crystal high

Any 6v receiver wires passing 7.2v wires? change wiring if so

Make sure cells aren't shorting on carbon fibre.

and finally, make sure no single hub is binding (I doubt this, what you describe sounds like interference)