It is your ABS activating.
Easy to prove, just remove the ABS fuse and take the car for a drive (dont worry, the brakes will still work, it's just the ABS that will be disabled). If the problem goes away, then it is your ABS that is faulty.
Probably one of the ABS reluctor rings is dirty, incorrectly fitted, loose, cracked, rusted or chipped.
Visually inspect them, it is often obvious which one is faulty. It should show up on a diagnostics live-data scan.
Remember to replace the fuse.
cheesy
December 2009