ClarkFlyer,
There are a few different things that could be causing your problem. The first thing to try though is to look at the receiver. Since you've already said you are going to try swapping that out, let's see what happens there. My guess is that's where your problem is. The vibration of your engine is what's causing the problems in your receiver resulting in uncommanded servo movement. A new receiver should take care of it.

This brings up a great point though. This is why all range checks should be done with your engine running. Just checking it without the engine running won't show any problems caused by vibration.

Hope this helps

Ken