I share dick's sentiments. Have a 88-740 that had similar stalling problems. Started with hot turn off and wouldn't restart until it had cooled for 1 to 2 hours. Turned out to be the Radio Interference Relay. The quick fix of resoldering the backside worked perfectly. Incedentily, I found this out by leaning on the Radio Relay by accident. The car stalled immediately. You may want to test it by removing the plastic cover and flexing the circuit board. If the motor stalls you may have found the answer (as I did).

Second one started two weeks after. Car would start and then stall (usually at idle but once or twice at highway speeds). Tach needle immediately went to zero when it stalled. I subsequently went through 2 Hall effect sensors thinking that was the problem (broke the first trying to install the devil.) Local Volvo dealer installed the second one for me for free (almost passed out from surprise). Problem didn't go away. Leaned on the car while a friend was over, pointing at the power booster. Touched the wiring harness and it stalled. Restarted the engine, touched the wiring harness at the power booster, car stalled again. Replaced both the power booster/connector and about 1 foot of wire feeding the connector and have not had any problem for 2 months.

Check all the electrical connectors and wiring harness first, second and third. Once you have completed the electrial connectors and wiring harness, do it again.