A 1991 volvo stres but wont stay runing

According to my wiring diagram, the headlight switch works a relay which is powered by two different fuses.
I would check the relay for voltage first.
You may need a repair manual to trace the circuits.

Does check engine light come on and if so it could be a sensor failing and if not i would suggest changing fuel filter and a good tuneup and if no help you may want to have fuel pressure test preformed which tells if fuel pump is weak and may need replaced....hope this information help and good luck please rate my post and it help more help is needed feel free to contact me...

Check your break fluid level. When the fluid gets below the sensor, the dash board break light will come on and stay on.

If starter fluid in the throttle body is making it run for a few seconds then you have a fuel problem. Either a new pump or relay or both.

adjust your idle screw to a higher rpm to stay runing

check overdrive or shift fuses. look in owners manual for location

You have an problem with your airbag system,Whats way your SRS is on the ,only way to find what is the problems,in the airbag system

Have you recently done any other work? Sounds like a vacuum leak (check lines) or maybe plug wires crossed? Good luck

The voltage regulator may be defective. Do all the guages work?

This sounds like a timing problem, try turning your distributor a little bit and the other thing to try is adjusting your ful mixture, but I would adjust the timing ( distibutor ) first.
Good luck and I hope that you take the time to rate this site.