1991 Volvo 740 Won't start in hot weather

[1991Volvo740]

Started having trouble with my 1991 Volvo station wagon last July when it began to not start reliably. Seemed as though it would start in cool temps (early morning/evenings) but not in warm temps (after sitting in afternoon sun). Now it's been at my mechanic's since July. Mechanic said it needed a new ECU and had it "listed" on 4 sites, so that he'd be notified when a used one came available. After TWO months went by, I finally got the part number from him; found a unit myself. (It only took 3 phone calls and I don't even know anything about this stuff!) The mechanic couldn't have done that? I found a friendly helpful place in FL that specializes in refurbishing electronic auto parts. They've been in the business for 20+ years.

So I get the part, mechanic says the numbers don't match. I send back their part AND my own Volvo's ECU; they say they'll repair mine if they've sent the wrong one. Turns out, they did NOT send the wrong one AND mine is NOT broken. Mechanic says they're wrong. They say he's wrong. He says there's "no ground from control unit to injectors, at times". They say "the problem must be in connectors, not the ECU". (They say they tested it for 45 minutes; they will test it again for another 45 minutes. He says they just happened to have tested it at a time when it felt like working.) Help!!!! I'm caught in the middle! I know nothing about cars! And I don't know why my mechanic was unable to find the part he thinks is needed, and I was able to find it with only 3 phone calls. And I don't know why he won't deal with the people who provided it.

Has anyone else had a problem with their Volvo not wanting to start in HOT weather? It's strange, I know! Thank you!