what's intriquing me is the airflow has remained constant (no clutch fan to slip) but something has deteriorated to where the same amt of air ain't no longer cutting it. Some tuning parameter could have been changed (you made no mention of that when asked). pump has been changed so if it was bad so no loss of water flow (unless the rad is partly clogged but as said it would for sure show up at higher RPM's if it was causing a problem). You have confirmed the temp with a mech gauge? Maybe it needs more radiator.
I think the variable is possibly 2 fold.
When I put this car on the road 12 to15 years ago everything was new . Engine was rebuilt radiator was new etc .. Now it’s got about 80 k on it.
the only other change was that at some point in the last 4-5 years I put a tstat In it. Never had one. I put one in to try and raise the temp over the road . The temp would sometimes never get above 140 ish.. well it didn’t really work as now over the road it will drop to 150-170 ish .
