9xxnick said:
Have your wheels been refurbished/repainted?
OP - this may not seem relevant, but trust me that it may be if your wheels have been repainted.
If paint has got onto the face of the wheel where it mates with the disk, this causes the clamping force of the nuts to be uneven across the disk's bell. When it heats up this allows the disk to distort - temporarily at first, then permanently as the "high points" get worn down. New disks will cure the problem temporarily (few hundred to a thousand miles, dependent on brake use) and it will then recur.
This is by no means the only mechanism that can cause brake vibration, but it is absolutely and demonstrably one possible cause.