I once bought a used Cat that had horrible steering. It turns out the steering arms had been disconnected and reinstalled improperly. If you push down on the front bumper and the skis toe in and out, this might be your problem.
If not, the next thing I would try is loosing the front spring in the rear suspension. with the rear end suspended, loosen the spring until it is loose in the holders, then tighten it up just a couple of turns