Jason,

I had a similar situation occur with my 88 240DL. It would just randomly shut off, like someone threw a switch. It turned out the electrical portion of the ignition switch was bad. I replaced it and I'v had no more shut down problems for the last two years. The switch back is held on to the keyed portion with two screws and has a multi-connector plugged into the back of electrical switch. It's some what of a PITA to get to the two small screws under the dash but aside from that, replacement is relatively simple. Try wiggling your ignition key while the Brick is running and see if it shuts down. There may be other reasons for your problem but a well worn ignition switch can cause this condition. Hope this helps.