The Bounce

[ampico-kid]

I've got another problem with my 30 Town Sedan that I purchased last July. I had a new Burtz engine, flywheel, cam, and head installed and just got the car back several weeks ago. I've been sorting through problems not related to the engine, one of which is a bounce. It had this issue before the engine was replaced, so I know it's not engine or flywheel related. When the car hits a speed of approx. 15mph to about 25mph the car bounces up and down causing the passengers to bounce up and down with it. Once beyond 25mph it smooths out and seems to be fine. Thinking that it might be related to the tires or rims, I swapped all four tires/rims from my 30 Coupe to the Sedan, and got the same results. If I'm coasting and step on the clutch, still the same bounce, so I'm guessing it must be something in the drive train between the transmission and the rear end of the car. But what? What should I be looking for? What can I do to attempt to localize the problem? Point me in the right direction please.