Same problem here a few months ago, first turn key halfway and see if you can hear fuel pump in back seat. If its fine then go ahead and ONLY replace fuel pump relay.

1994 855 5-speed 155,000.

Have experienced the same problems with my 94 850 over the past summer.
The car typically would not restart after a short trip. Give it a couple of moments and it fires right up. Occasional stall while driving - again during short trips, after the car has heated up. Problem is most frequent during periods of high heat and humidity. (Does yours go into high rev's on start-up?)
Cleaned the idle motor, mass air inlet, replaced fuel filter, ran injector cleaner through, changed gas stations, etc., etc. - nothing helped.
Finally got so po'd that I bought an OBDII interface for the car and using Scantool software came up with following diagnostic,
"closed fuel loop, O2 sensor fault". Last week my "Lambda Sond" warning indicator light came on (green triangle with a greek symbol inside), ran another diagnostic and came up with the same message. I did use the Scantool software to reset the chip and since have not had any stalling issues although I now have the "service engine" warning light come on for @ 1 minute every time I restart the car. Also it's been cooler...
Given all that I've gone through so far my best guess is that it's time to replace one or both of the O2 sensors on the car.
Good luck, and let me know if you find your solution.
Armie
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Imagine all the people...

Next time it stalls, turn the key to the accesory on position, don't try to start the car, see if you can hear the fuel pump pressurize. If the pump isn't pressurizing, it is your fuel pump relay. I know on the 93 models, and i believe still on the 94 models, the Fuel Pump Relay wears out. I had a stalling problem on my 93, where it would die and then take a few minutes before it would start again. Turned out the be the fuel pump relay, about a $60-70 part and less than 5 minutes of time to replace yourself.