I would strip the car to the bare kit peices.
Put the lower deck on a very flat surface and each time you add a part check to see if the chassis tweaks. if there is not tweak throughout the build then it HAS to be something to do with the drivetrain. Make sure your diff pulleys are shimmed correctly and not rubbing on the bulhead or shock tower. I once had it where i shimmed to pulley too tight and it did a similar thing. You can usually tell if it is too tight because it is a bitch to put the top half of the bulkhead over the plastic cams.
other than than, check all the driveshafts are working properly,check all bearings are working check wheels are not rubbing anywhere, you have take into account that if something is close to rubbing, when you go round a corner, things move with the weight transfer. Wheels move quite alot also.