Phil, first thing I thought when reading the subject line, before opening the detailed post, was a dirty intercooler. But then I read the story. No mention of intercooler. And thorough attention to the radiator, which was the trouble when I had a similar symptom.

It is scary when you're well away from home and that gauge starts climbing above 9:00. I borrowed my daughter's car that day, because the AC worked and the trip was to be a tour of Smithsonian exhibits, five of us in the car on a hot July day in traffic on the DC Beltway.

We tried the 440 AC substitute, and running the heater (it helped), but besides my anxiety about possibly needing a long tow and ruining the day, we were all sweating.

Best part of the story was seeing all the crap come off from between the fins when I got her car home and turned the hose on the radiator and condenser. Didn't need to replace the rad, just clean it outside. I know this thread has a different outcome -- I'm hoping the fresh rad will be the trick, but Phil, you had me there with 255 AC. Out here the humidity and traffic congestion suggests 440 AC.
--
Art Benstein near Baltimore

"...people will always choose a simple lie over a complicated truth, because the lie has one unbeatable advantage: the truth always has to stick to what actually happened, whereas the lie just has to be easy to believe" - Fredrik Backman