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 .

Had the spring for the flapper valve in my exhaust manifold break and the valve wouldn't open. It cause all sorts of hot engine issues that took me forever to sort out.

If you have headers, probably not the issue, if you have manifolds make sure that valve is working and will open.

That’s something I wouldn’t have thought of.
Valve was removed

Trust me, I didn't think of it either. I found it by accident.

This was my 87 Diplomat I swapped a 360 into. Anyone that's ever seen the stock exhaust setup on that thing knows how well hidden the flapper is by the pre cat.

I replaced the fan clutch, water pump, belts, thermostat, cleaned the block with old school cooling system cleaner (the two part type).

I even found out that I could put the 87 pickup radiator, with HD cooling, into the Diplomat if I used the truck lower hose.

All of that did diddly and I was dropping the exhaust to check for pluggage when I saw the valve was loose and flopping.

Fixed the flapper and it all went away.

It took months of trying to figure it out.

retard timing will make them heat up also.

make sure you have the correct # of vanes on the WP impeller

a/c wp = 6 vanes
non a/c wp = 8 vanes

a/c has 6 vanes with smaller pulley(spins faster) and pumps x amount at idle

non a/c has 8 vanes with larger pulley(spins slower) and pumps same x amount at idle

mixing the 8 vanes with small a/c pulleys can cause pump to cavitate at higher rpms and temps go up. or a 6 vane with large pulley it will not move enough water at idle.

make sure the lower hose is not sucking flat = add a spring inside lower hose.

make sure the fan clutch is spinning at correct speed all thru rpm range or just replace it with a new HD thermos clutch

just a couple things to check that I did not see mentioned.

good luck