I had a similar problem with my '85 380 SL. When I first purchased it the ABS light on my dash was constantly on. My mechanic found the problem was some faulty speed indicators in the wheels (I know that is not the proper term for the part but it is the only discription I can think of.)

It cost me about $125 for them to test out each of them and then another $300 in part costs to replace them (They were already doing alot of work under my car so they installed them free of charge).

I can go home and look up the work description to give you the proper name of the part they replaced if you'd lik.