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.
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39 M54, e53 N62 & Tribby