My cars are really old . When I experienced hot under load (speed or uphill)
but dropping to great temps in idle or cruising I immediately assumed bad flow
of coolant and needing to clear internal blockage.

In my case it has always been blocked airflow through my old radiators.

Turns out I never needed a radiator replaced and just need to clean the
fins and dirt, debris, and hair like material clogging up the radiators.

Hair never biodegrades, so this has it's own problems.

There is a radiator cleaning product, but I have experimented with hair clog melting drano/drain cleaner. The stuff really disssolves hair.

Lots of shop vac in blow and suck mode, compressed air - I eventually got
the fins really clean. And the radiator cools as it should.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250