Check all steering parts by having an adult helper familiar with tractor in seat with engine running, have them turn steering wheel all the way right then all the way left while you're looking for excessive movement in steering components. Safety First have back wheels scotched. Back in the 80's I was working on a GMC Astro 18-wheeler tractor that did the same thing while holding steering wheel straight it would turn right. I soon found the problem, left front spring eye bolt bushing was broken. Let us know what you find, thanks.
