When my 1994 volvo 740's engine warms up the car will sometimes stall. I brought it to a repair shop, the filters were replaced. And it did worked. But weeks later, i'm again dealing with the same problem. It's not the thermostat or the spark plug either. Hope you could help me with this.

 

I had same problem on my Volvo 740. In my case, it is a choke valve. The valve didn't open when warm because the sensor is not sending a signal back to open again. The problem was solved when i replaced my sensor which is not really working normally. You can check both sensor and valve for a good start.

 

Does your car have a engine speed sensor?
How old is the fuel pump relay?