Honestly you don't want to mess with asymetric geometry.
You should start to check that there is nothing's bent. Then you should check the bearings: if you have one of them with is crappy, you could have some strange reaction when driving. Finally you should check you rear differential to see if everything is in place and if it is working properly.
Are you using the servo which came with the kit? This is a entry-level servo, it could be worn (electronically) or pinions inside can be out of order.