it has a stering wobble at 40mph? ive put 18in wheels one, 4 months old brand new, the wheels have been balanced 3 time 2 at the place where i bought the wheels and they say that my passanger front and my drivers rear are slightly buckled ( and they wont rufund me, they say that pot holes could be the course of the problem) anyway could you advise me of anything else that could be the problem ,,,,, the wobble, you could call it a very bad wobble, it comes on about 40mph slows down at 50 then it back at 60mph ,,,,,,,,, i would be very greatfull if you answer this question for me ,,,,,,,,,,,,,,,,,,,,,,,,,, paul

Check the wheel nuts. Maybe 3 of them are not on tight? I did it in my car, changed wheels and then drove the car about a mile and only noticed there was a problem when I made a corner.
Other than that, you would need to go to a wheel alignment shop and have them check you steering assembly for worn out tie rod ends.

Sounds like it's the tires, rims, or a balance issue. It wont be anything else because you said it comes at 40 slows down at 50 and starts again at 60 so it is 100 percent related to those tires/rims.

I'm with him above, but the bead on the tyre lip could also be broken - happened to my father once rare but does happen.