Need transbrake help!
 

I have a powerglide with a transbrake in my mustang. After I stage the car I hit the transbrake button after the tree starts coming down. When I step on the gas to bring up the rpm's the car jumps forward about 4 or 5 inches then stops. I let off of the button and then it launches. Anybody had this kind of problem before and if so what did they do to fix it? It is a Coan tranny. I would like to get it fixed before the next race. Is it possible to use the linelock also to keep it from moving forward? I do not have them wired together.