I had the exact same problem and still haven't figured it out. Check out this thread where I asked about it.
http://tinyrc.com/forums/showthread.php?s=&threadid=792
I got some great advice, but no love with the fixed turning. Its a moot point now since I was working with it and I ended up accidentally breaking one of the tiny copper wires that leads to one of the electromagnets. Ironically I was trying to insulate those wires in case that was causing the steering issue. I'm wondering if replacing those wires with something higher gauge (and insulated) would improve performance in general along with solving the steering problem? I noticed that while I was having the steering problem you describe, the magenet itself seemed to be more attracted/repelled (not sure which?) to one side than the other when tested independantly. Either way those spider thin wires made me nervous in the first place.
Has anyone here replaced those magnet power wires? The connections to the circuit board look tenuous at best so it might be difficult to solder a new connection on.
Anyway hope this helps yet is not disheartening.