Dude my wife has been driving the same way, always looking for a safe place to die. For two weeks my car was at the shop. They used the 88 SEL for an errand car for the whole time and it ran great. They gave it back and the thing dies - Dead. The tach goes to 0 immediately, all power stays on to the dashboard lights. The car will turn over but not start, until a while later, then it will run fine - for about 10 miles.
I took it to another shop ( after I replaced the OVP and the fuel pump relay - which made no difference). He found that there was a loose connection in one of the plugs for one of the relays. He tightened up the plugs for that relay and told me to sell the car. He says that most of the electrical connections on this car are very similar and that they get old and loose. That is his solution.
I am now looking for a newer Mercedes with different connectors.
Anyone looking for a parts car?

Good Luck - Scott