When my '86 420 did this (repeated stalling) a replacement crankshaft position sensor did the trick. I do not know what the fuel pump relay was doing but it would almost always start right back up and drive a few more miles then stall again. Once it cooled off (I got to the wedding) it ran fine on the return trip, until it got hot again. Of course it might be something like a bad ignition switch or a bad ground on the battery cable, or something else. Lots of people on here know more than I about this.
While you are at it, take the ignition control module off the driver's side inner fender; underneath you will find some white stuff, probably dried out. This is silicone heat transfer paste, it helps the module use the inner fender as a heat sink. Scrape/clean it all off and replace it with some fresh. You can get this at Radio Shack or an electronics store. It will help the ignition module live longer.