first thing, make sure you have enough fluid in it...
if that doesn't work, try flushing out the tranny.
Was happening to mine between 2nd and 3rd...and if i went around a corner while locking to the right, it revved and then sped up. The above worked for me...
Mine only does this occasionally when i now start the car and drive it before it warms up a bit. Strange, but cant afford to sort that out now