intermittent abs light, it mainly comes on when going over a large speedbump.

the ABS light will be off, until you go over a large speedbump, and then will stay on for the rest of the journey until turned off. i recently had the coilover springs taken off, cleaned and adjusted properly by my mechanic and the two front tyres replaced, the light didn't come on for ages after that, and i thaught the problem was fixed. i have been told it could be a faulty sensor on on of the wheels or that there is a loose connection somewhere. Any advice on this on this would be much appreciated thank you.