I recently had the same problem. I have a 91 300 4matic. It has 255,000 klm and I just had the engine re-built. To my surprise, the same thing happened. It felt like I was running out of gas. There was no power. I made it home and made some calls. After checking it over I checked the distributor because plugs, wires everything else was new. Took the Distributor cap off, unplulgged the wires and the rotor had fallen out. The three screws holding the rotor in place had come undone. If it was a couple of days or even weeks I would think maybe they were not put on properly when the engine was rebuilt but a couple of months....

It's very simple and not time consuming. It may take you a half hour. Just make sure the screws are secure.