This sounds an awfully lot like my problem. I have a 1994 E320, with 148k on it. The problem started a week ago when I was driving. The interior lights came on, seatbelt sign flashing, and my radio stopped putting out sound but stayed on. I got out of the car and my power locks did not work. The problem happened about once a day for a couple days until I took a short trip to the grocery store. Came out and car wouldn't start, just like you were describing. I waited for a couple minutes. Got out, wiggled some wires under the hood, tapped on some things, and then it started up just fine.
I took it to my mechanic the next day, but only because I slammed my hood while my toolkit was still in the engine bay...broke my antifreeze resevoir. My mechanic didn't have a solution, but he checked to see if my ignition switch was loose (it wasn't). Apparently (I say apparently because I haven't talked to him since Friday, they still have my car) they poked around, replaced some fuses, and looked behind the radio at the connections. They didn't find anything odd and were unable to duplicate the problem. They are replacing the starter and the ring gear to fix another problem I was having. I hope that it fixes my electrical problem as well.
But, it's sounding like my ignition switch is going to need replacing soon. I'm glad, in a way, that someone else is having the same problem as me... let me know if you get it fixed.