On rare occasions I have seen engines starting to run backwards by themselves. The last time was two weeks ago when we were testing a new MDS .68 engine in a trainer. It was idling, and all of a sudden it was going backwards. When that happens you just stop the engine, and start it again. That usually sorts it out.

I have no idea if that is what is causing your plane to go backwards, but it is another idea at least.

Hope you get it working!