You should get Scott Stoops book on the mechanics of it. he starts writing for FlyRC now.

But to answer the question, because you're at stall speed, the torque of the motor becomes a factor now (esp with large props). The plane wants to torque left, that's why you have to correct with a slight right aileron. Once you figure out how much aileron your really need, your plane will wing rock much less. This is also why when you throttle up and down, you get a rocking sensation with the plane (which is one reason why I prefer to find the right torttle and stay with it, instead of gassing up and down). The other corrective input is that (I think) the P Factor will make your plane yaws to the left on upright and right on inverted, which is why you should add a little bit of right rudder (upright), or left rudder (inverted) to keep the plane flying straight.

Try correcting for this on your ailerons and rudder, and you'll be set