I have an Aeroworks 540T ARF that does the same thing. I have been in this hobby a very long time and owned a lot of airplanes but this one stumps me. I have messed with this rascal quite a bit and it's about as close to a perfect setup as you can get and it still does it. If your radio will handle it you can mix in a little bit of down elevator at low throttle to get rid of it but it frustrates me that I can't figure it out. The plane flies great so I finally said the hell with it and just go have fun with it.