This sounds a lot like what our 1989 190e did (similar engine, same CIS-E fuel injection).

It would start, then stall. Eventually it would run. Sometimes it would only keep running if the revs were kept high. It became undrivable.

I replaced the cap, rotor, OVP relay: all wasted money.

The problem was the potentiometer on the front of the fuel distributor / airflow meter. The carbon tracks in the potentiometer tend to wear out, especially at the idle position (our 190e had 200,000 miles on it). When the potentiometer is faulty, it sends a spurious signal to the computer, which then has a nervous breakdown, and can't make up its mind how to control the idle.

The part is no longer availablefrom Bosch, or anywhere, it seems. They will sell you the whole unit for $1000 plus. If you look in the archives, it seems that this was quite a popular repair, before the part became unavailable three or four years ago.

Solution: a new fuel distributor / airflow meter unit, from a wrecker, off a low-miles 300e. Even then, the EHA needed tweeking (delicate adjustment of the tiny allen-head screw behind the brass plug), to get the right pressure differential between the upper and lower chambers of the fuel distributor. Now it runs beautifully.

This all became apparent only after I made up adapters, for three meters at once, to measure EHA current, idle valve on-off ratio (dwell), and the voltage on the potentiometer. And, of course, I eventually neede the CIS-E fuel pressure tester to measure the differential presure.

Good luck!

Andrew