I have a '91 300-SEL that may have the same motor as yours. I may stand corrected, but mine is the 12 valve, single cam M103 engine.
I went through something very similar to your situation a few years ago and it turned out to be a faulty fuel pump relay. In my '91, that function is housed in a MAS controller that costs around $500.
The next time this happens, do the following if it won't restart immediately.
Find the MAS unit. It's generally silver and a bit larger than a pack of cigarettes and has a black knob on top of it. Located on drivers side near firewall. The black knob is rotated counter-clockwise, then the unit is pulled straight up to remove. Jumper pins 1 & 2. Those two pins are the closest to the left side(driver side). You may here the pumps run. If so, try to start the car. If it starts, you found your problem, but good luck with the $500.
This is just one more thing to consider.
Here's a pointer to online MB documentation.
http://mb.braingears.com
And here's what the MAS looks like:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1KU172UOY1UM1DWA7I&year=1991&make=MB&model=300-E-002&category=P&part=MAS+Control+Unit