I went through several front tires of different brands thinking the tire was causing the wobble. Every time, a new tire would seem to fix it for around 1000 miles, then it started again. It turned out to be the steering head bearings. Since upgrading to tapered bearings it's still rock steady at any speed after roughly 10,000 miles. I can't say that's what's wrong with yours, but if i was a betting man...
