Could be the stock wheels, unless you've had other, similarly sized tires on before without that issue. If so, then I'd guess one of the tires had a cord separation inside the rubber and will have to be trashed. <img src="/forums/images/graemlins/scared.gif" alt="" />

Whenever I have to put more than about 3 oz. on to get it balanced, I return the tire. If I ever bought custom wheels, I'd always do it locally because even though I'll pay more, I can check the wheel balance before ever putting a tire on them and return the ones that don't balance well.