Mercedes: 420SEL Engine Suddenly Dies, fuel pump relay, correct diagnosis

Question
I  have an intermittent electrical problem with my 1988 Mercedes 420sel.  Car has approximately 130,000 miles on.  

I recently started having an intermittent problem of the car suddenly stalling.  You will be driving along, or stopped at a light, and suddenly, it's as if somebody flicked a switch, the car engine will stop.   There is no pre-warning such as faltering rpms or stumbling.   Car will start immediately afterwards after about 5-10 seconds of cranking.  Normally the car starts with 1-2 seconds of cranking.  It tends to happen intermittently and when it does it doesn't reappear that day.
The few times it happened the car was fully warmed up.

Apart from this issue the car runs fine.  Idle is generally smooth though on occasions there is a slight lumpy  feel  which goes away if you give a squirt of gas.
Acceleration is smooth with no hesitation.

I've noticed no other problems with electrical system or with engine.  

I've been checking the Mercedes forums for answers and the two most plausible answers seem to be either the fuel pump relay or the overload protection relay.

I'm wondering if you can give me your opinion as I don't want to spend a lot of bucks on just replacing parts as these relays are somewhat pricey.

Thanks. in advance, for your help.  

Answer
Answer:
Either of these could cause your issue.

I suggest you take it to a local independent MB shop for correct diagnosis.
There are several other possible causes.

Possible local MB shops
http://www.mercedesshop.com/shopforum/forumdisplay.php?f=14