I have a 1997 volvo 850 that has started intermittently stalling. It usually happens when i’m idling, like at a stop sign or traffic light, but recently it did it on the freeway. I have taken it 6 times to the mechanic (5 times to one and finally having learned my lesson the last time to a new one) and they say it is one of 4 things, but can’t figure out which of those 4 things it is until it stalls on them. which it hasn’t done. it acts perfectly when at the mechanic’s and then they always give it back to me, only for it to happen again. the last time the car ran beautifully for 6 weeks before stalling out on me again… oh, and once they replaced my fuel pump which seemed to work for a while.
I’ll bet one of those four things is the ignition module. If the ignition module is the cause, it will steadily get worse and worse until there is no doubt. If you don’t mind throwing parts at it, you could replace that and see if the problem goes away. It could be kinda pricey, tho. Last time I replaced one for a Volvo, it was $150.
Similar symptoms in a '98 V70XC (5 cylinder, turbo motor) was the MAF, or mass airflow sensor.