Volvo: intermittent stalling 1989 240DL Volvo, fuel pressure gauge, ed gomes

Question
I've been having stalling problems with my Volvo for the past six months or so.The car will stall sometimes when I'm driving(hot or cold),other times after I've turned the car off and go to turn it on,it wont start.After waiting five minutes or so the car starts again.
I have taken the car to a mechanic who left the car running for a couple of hours to see if it would stall so he could diagnose the problem, but no luck. I've tried methyl hydrate(in case there was water in the tank),replaced the rotor and plugs,checked and replaced fuses, and still the car keeps stalling.
Before I start replacing fuel pumps and running up a expensive mechanics bill, could you advise me if this might be an electrical or fuel supply problem? I would be most grateful for any help you can provide.
Thanks, Ed Gomes

Answer
Ed,
You must run some tests. Install a fuel pressure gauge and see if when the car stalls you have a loss of pressure. Read any codes in the system and fix them.  Check the fuel pump relay for burnt contacts or a cracked PC board. Pull the relay out of it's case to inspect. If it is original you may decide to replace it.