I'm not sure if this is your problem, but let me tell you about mine and you can check this out on your engine. My dad gave me his old 1985 740 Turbo with only 77k miles on it, and it was kept in a garage most of the time. Excellent condition but it has not been driven much in a while. The engine was shutting off sometimes at long traffic lights; however it would crank right up again. I noticed as the engine was starting to slow down one day that the voltage was going down before the engine shut off. I put in a new battery, but that was not the problem. Most american cars have a postive battery terminal with the wires molded into the terminal connector, and are resistant to corrosion and wire oxidation. This Volvo does not. The alternator connects with a large gauge wire to the starter, and the starter with a large wire to the battery. Two other wires on a crimped connector at the battery are the feeds to the dash in the car. This powers all the lights and electronics. The crimped connector had oxidized internally and was a high resistance connection. Therefore as the brake lights and headlights sucked up the power, this high resistance connection was causing the car to shut down. I cut off the crimped connector and about 1 inch of the wire. I installed two new connectors in place of the one. They don't fit quite as well on the battery post, but are very secure and electrically good. Now the voltage does not go down at all when there is a load, especially when the brakes and backup lights are on. It has not shut down since I did this.

Now I did find another problem that could cause a shut down, but this was not my main problem. I was wiggling all the wires in the engine with it at idle. When I touched the air mass sensor's connector the car shut down or would vary the idle speed. At first I thought the wiring was bad but it ohmed out fine. Upon further inspection, I notices some black oxidation on the pins for the air mass sensor. I used to see this when I fixed computers years ago. This stuff is hard to see and has a very high resistance. I glued a piece of 600 grit sandpaper to a small piece of balsa wood and wet sanded the pins with light oil on top and bottom on the pins. Then I cleaned the pins and put a very thin coat of lithium grease on them. Now I can wiggle the connector and it has no effect on the idle speed.

Now I am not a good engine mechanic, but I do know electronics and I am sure this fixed my problems. This may not be yours, I did do a lot of other things i.e. new spark plugs and igitions wires,etc. before I found these problems. The issue is that these kind of problems are not that easy to pin down so I would definately check those two things. Good luck and I hope this helps. I might have to get someone to help me out with my oil leak, but my engine is running great now.

regards,
Randy