iceman,
Is this a new MBX5 ProSpec buggy, or an older used kit? The reason I ask is that Mugen has made a lot of running changes on the car. The latest E0109B front uprights on the MBX5R have much more movement (more steering) than previous revisions. Check your steering end-points (left and right) with the arms level. Aside from that, make sure you are running 1* front toe-out (that is 1* per side). Then follow the rest of Jason's setup. It is a great starting point.
Good luck,