Check Out That Chassis
There are a few things that will make your car dance round in the shut down area; I would start by checking the caster angle on the front spindles. You want at least 5 degrees, then in the rear you need to look at all your rod ends. A little play in each one adds up to big movement under a load.
Your chassis guy may be right if your wish bone is light duty and long
enough it will flex and this adds to your movement. If your panhard bar was installed properly it will work great for a ladder bar car and perform just as well as any other centering devise. Don't be afraid to go back to a tried and true part sometimes they work better then the trick stuff.
President, S & W Race Cars