i'm 19, my 89 240 was given to me from my grandpa after he was fed up with it.
Same thing, warm stalls, bucking while cruising, lots of cranks to start it.

old man Gill took it in to a few places, they diagnosed it as fuel related problems (turned out it was only the fuel relay) and after changing fuel filters, fuel pumps, etc. it still wasn't running strong (including changing the relay)

So he bought a new S60 and gave the 240 to me to try n fix. He told me the spark plugs should be fine but i pulled them anyway only to see they were disgustingly corroded, and also the wires were shot to shit. After new platinum plugs and new bosch wires, cap n rotor, I've never had it stall, no bucking, starts immediately when i turn the key. I plan on cleaning the flame trap soon too, as i think its something like a PCV valve? i dunno, but i'll get to cleaning it soon. I also ripped out the cold start air tubes and sealed shut the opening in the air box so its fed only cold air. My 240 had around 215,000km when i got it, since i've been driving it its really grown on me and i plan to keep it around for a lot longer.

I never like taking a car to a shop for them to diagnose, no one nkows a car better than the person that drives it (unless they know their shit, apparently not the volvo dealership by my place) I recommend you start with the easy stuff first,

plugs/wires cap/rotor flametrap cleaning check your airbox to see if the internal thermostat has the cold start opening stuck open.

If that doesnt work it could possibly be a fuel related sympton, o2/crank sensor..?

I find this site awesome for troubleshooting, diagnosing and there are also step by step instructions for doin it urself, just take the time to read. Or if your son has any mechanical inclination let him deal with it, thats what happened to me :P


Good Luck,

Amar
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'89 240 All original, custom exhaust, KnN, custom guages, 260,000kms