Did you do the range check with the engine running. If you have any metal to metal connections, ie. clevises to engine throttle, remove themand reinstall with plastic/nylon to metal. also make sure you tape your receiver crystal in, so it can't vibrate loose.

When you do your running engine range check, advance the trottle up and down, to make sure you have good smooth throttle control.

I think everything else has been addressed. The other thing addressed, but I didn't hear a good response to, was if your antenna is running through the fuselage, you may want to reroute it outside, either on the bottom or top. Make sure you CA a piece of silicone tubing in the hole, exiting the fuselage to pass the antenna wire through. I have had similar problems on serveral planes and re-routing the antenna cured it.

Good Luck,
Mike Boyd
AMA 7197