Check the wiring harness for degradation....look at the ground/brown wires. Raw copper/no insulation indicates the harness is breaking down. Generally unrepairable...have to replace the entire engine harness.

Next is the ignition amplifier. Next time it stalls, try a new one or borrow one from a similar volvo...try a used one from a junkyard. Or replace yours on GP and maybe it cures the problem.

Also check the wiring/socket/plug to the distributor and/or the Hall effect sensor (I use a graphing DVOM or you can use an oscilloscope but oscope not really useful on the road!) signal. Maybe it's something as simple as a bad/intermittent connector at the distributor (even the side of engine ones have the problem, so do the camshaft mounted dizzies.

Or get a can of freon or freeze (try radio shack) and next time it stalls goosh/freeze the ignition amplifier and see if it starts up. Perhaps the IC is experiencing thermal runaway.