I had the same problem. The load of the brake lights in combination with low RPM would make the voltage go low, and occasionally shut the engine down. Take a look at this pic

http://mywebpages.comcast.net/wduke6/volvo1.JPG

This is a picture of the wires going from the battery to the dash of the car, i.e. it feeds power to most of the electronics in the car. The battery post was clean. This terminal was clean, but the wire itself is green due to corrosion, and is in bad shape under the crimp. This is a high resistance connection. It is mentioned in the FAQs , and some folks have fixed it by soldering it. I would not attempt to do that if it looks as bad as this one because my wire was badly oxidized. It probably got acid on it at one time. I just cut this much off and got to some good wire. Then I put some new connectors on it and the problem went away. You might want to double check to make sure you don't have this problem too. Also make sure your ground wire on the alternator looks good. The new volvo part is much larger gauge wire, and only cost about 5 bucks. Mine looked bad where it attaches to the engine. I think it is isolated from the car frame because of the rubber bushings so the ground wire is needed.

regards,
Randy