89 300se stops at low speed
 

Runs rough at low speed- stops at random-difficult to restart

Most likely it will be a failing fuel pump relay .Its misleading , seem like a ignition fault, but of course the engine restarts.
In my 300se i have found that by replacing the FPR and a fresh set of fuses sorted all the intermittant electrical problems.
I assume you have a new fuel filter and fuel accumulator .

I would change distributor cap and rotor. Get a Bosch. Also you can not test the cap and rotor under load, at least I can not.

This is unless you very recently did change cap and rotor.

Possibly crankshaft reference sensor, but I'd pass on that for now and do cap and rotor.

Edited
If you have a spare MAS relay you can swap them out and see if that's the problem.

Shouldn't be a MAS relay on an '89.

Be careful with Bosch. There are some el cheapo erzatz "identical twin" Bosch leads and caps the come from the Middle Kingdom.
I would recommend OEM - Beru.

300SE:my 89 has the fuel pump relay ,MAS came following year on for 103.