Your car is old enough to have the "rotten battery cable" problem found in the FAQ. I had the exact same problem. The engine would die sitting at a light, but would restart immediately.

http://home.comcast.net/~wduke6/volvo1.jpg

The wire under the connector gets corroded to the point it is a high resistance connection. If you wire looks like the one in this picture, you need to take care of this problem even if the engine cut out is being caused by something else. Also, If you never replaced the fuel pump relay you should take care of that too like one of the other posts mentioned. The ground wire on the alternator is bigger than the ones they installed in 85. I would replaced that too if the insulation looks brown on either end.

Good luck,

R Duke