Yes! 3rd rudder, whatever you want to call it. this could fix the symptoms. and allow you to run straight, but it may cut down on the top speed (it may not be noticeable). Just be warned, as I said this may fix the symptoms, but not nessicarily the problem. Chris may be on to something with the motor mounts.

I had a similar problem with one of my boats; suprisingly it was a pontoon style(they usually run really straight, hard to turn). After trying everything I could think of, I finally replaced the pontoons with new ones. Viola! Something I could not see nor detect was unbalanced. It was a radical solution, but it worked. Keep trying. You will find a solution!
Sean