I've dug thru the archives on this issue before and came up primarily with 3 things. There could be more I suppose.

a. faulty OVP
b. faulty ABS wheel sensor(s) or maybe rear sensor in the differential.
c. faulty alternator - maybe not as likely, but one member here indicated his tech attributed it to a bad alt. diode that caused a wheel sensor to be or appear to be bad.

Are the brakes kicking back when coming to a stop? If so, you could have real ABS issues and that would rule out the OVP.

Lots of discussion to be found using the SEARCH facility.

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'91 300-SEL