I had this a little while back. My ABS light would come on randomly but, while the system was technically working (light off), the ABS was cutting in far too early, especially at low speeds. A dodgy sensor was eventually found but I've had that before and that normally just kills the system until it's fixed. On this occasion though, it was also found that the brake pads were well past their prime (in fact they were utterly shagged, really should've spotted it earlier :embarassed:) Now it may be coincidence and I'm not sure if it's technically possible for excessive pad wear to baffle the ABS but it might be something to check.
S.M.