My son's 244 Volvo 1980 had exactly the same symptoms as yours. Luckily, it was quite noticeable during test drive, so I could bargain hard with the seller. Before agreeing on the purchase, I took the car to a friend's garage, where they quickly found that an air-conditioning switch wasn't operating. This was a pre-electronic car, and an pneumatic switch was intended to increase the idle speed whenever the A/C started to operate.

I've paid a very low price for the car. The mechanics have replaced the switch and the car has never-ever stalled again. I've sold it reluctantly 18 month ago, but we can still it around the suburb. The old stinker was a mechanical marvel, compared to my corrent 850-1996.

With more modern cars, I would look at the sensors which control ignition. Oxygen, speed, etc.