I don't think your OVP relay is the problem. It would cause intermittant not regular ABS light malfuction. At the speed you are getting the light the system does a self test on the wheel sensors. You undoubtably have a wheel speed sensor problem.
I'm guessing that you have a spark plug resistor problem for your running problem. They result in intermittant single cylinder misfires. They are easily identified with an ignition scope.
Bosch Master, ASE Master, L1
33 years MB technician