My '93 850 (407000) has been amazing but is giving me issues these days.
About a month (maybe two) ago the car started to stall very occasionally while I was driving but only
when I was coming to a stop or stopped at a light. This was occasional and the car would usually
restart... it might not stall again for a week or two at first... it got more frequent and the delay longer for the car to restart... it would crank fine but no turnover... could hear the fuel pump/delivery activate etc but no start... then after a delay of 5 or 10 or 30 mins. ... I turn the key and it starts and drives fine...
So I had a similar thing happen to me with my old 740 and it was a fuel pump problem... replaced the fuel pump in the 850 last week... good for a few days then same issue... read another thread on the board about another guy having stalling issues with his 850.. suggestion was to replace fuel pump relay... worked for him... I waited until the car stalled while I was driving it to see if that was the issue so of course it stalls while I'm at the coffee counter at a drive through ... wouldn't restart... got a push into the lot and put my alternate fuel pump relay (pulled one out of another 850 - same year and color relay #103 under fuse box)
It made no difference... but car did start after a few minutes. I also noticed that the relay I removed was definitely warm to the touch... is that normal?
Did the computer code check a few days ago for fuel system (A-2) and got a 1-1-1.
Last night car stalled at laundry mat and would not start again... went back this morning and it started right off and got me home before stalling just as I turn into parking spot and would not restart.
When I was researching the fuel pump relay the parts list also had a fuel injector relay listed under fuel pump relay... anyone know the difference or anyone have a suggestion???... am at the point where local talent may not be adequate and I have to tow to Volvo dealer 30 miles away... expensive... can anyone help here or anyone had the same issue?? Any Help Appreciated... please.

fuel pump is definitely a concern.

I have also found that at this age it's very possible for the wires passing through the top of the fuel pump assembly to start to come apart. I replaced a fuel pump recently that didn't need to be replaced - the wires were all busted up inside, and the electricity wasn't making it into the fuel pump assembly.

The fuel pump relay is also a concern, but don't run your car with the relay jumpered. It's fine in a pinch but a long term solution. The relay has the 'prime' and 'emergency shut off' functionality - you want both of these things.

Also of concern however, are the condition of ALL vac and PCV lines, as well as your trans fluid (may not be downshifting/unlocking torque converter properly).

Get a handle on any possible vac/PCV leaks before going much further.

--
1998 V70 AWD->FWD Turbo 200k+

I don't like the 'try from another car' method on the fuel pump rely you've used. You can jump the relay (i can't remember which pins but i'm pretty sure its top to bottom) and verify if its working or not...

I'd try jumping (maybe its 15 to 87, but google it to be safe..) the relay and make sure its working... Next up is testing fuel pressure at the rail when it won't start. if its got pressure then the injector relay could be at risk but you'll have to verify its not spark/compression. I can't remember if you can jump that one or not