When I bought mine, it had been sitting aorund for a couple of years, and althought the tyres had good trea don them, I had quite a lot of wheel wobble over 70mph. I took my car to a tyre shop to get the wheels balanced and the guy told me not to bother. He said if a car's been sitting for ages then the tyres get flat spots, I guess the rubber or silicone compound in the tyre 'creeps' or flows rather like a ball of plasticine (but slightly slower), and you end up with a non uniform weight distribution. So for me the first thing I'll try is new tyres when I get round to forking out for them.
Andy