brake discs keep 'warping'

[bechdan]

Ive got a recurring odditiy on our dear old yaris, Ive replaced the front discs and pads 3 times in the last 3 years. Starts as a mild vibration on braking then gets progressively worse with a loud tapping noise up to the point its hard to hold the steering wheel when braking hard.

All the bolts are tight, the sliders and piston have been greased and the seals are intact. Ive cleaned the hub/flange with a wire brush so the disc sits properly. Ive tried two different brands of discs/pads.

Any other suggestions of what could be causing this?