Probably time to re-build the brake calipers.
Had the same problem, and also a shimmy at +/- 70 mph. New rubber seals + cleaning the piston calipers (which started to corrode at the outer part) fixed it. You might want to do rears as well, because if the front rubber boot for calipers are in poor shape, the rears are not far behind. And rears a PITA compared to the fronts.