ABS WARING LIGHT PROBLEM

On my '84 500SEL, I have recently had the ABS warning light come on. I pulse tested the front sensors and they checked "good". The car is not specific as to when the light will come on. It can drive for an hour at 75mph and then the light will trip or on other occasions the car can go for 5-10 min at 40-45 mph and then come on. The light will reset when the car is turned off and then restarted, or it will rest when the OVP relay is removed while the motor is still running.

Could this be that the OVP relay is the original from '84, and should it be changed to the newer version, or is it something totally else.

Please help.

Thanks