This is too strange. My daughter was driving my '86 300E two days ago and it did exactly the same thing, except that the tach surged and it would not start after she parked it (low battery).
My overvoltage relay is new. Its function seems to be to protect all those delicate little relays and control units from too much volage. It is located behind the battery. It is silver with a fuse on top of it.
I replaced mine a couple of weeks ago in an attempt to get my ABS working and because it seemed to be mentioned in connection with just about every electrical problem. It costs about $45.
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,