Lots of good suggestions...

Reverse-flushing might help - If it does the block needs flushing too;
that's where the crud is coming from... When seen, one often finds
the wrong coolant has been in use, or change intervals not observed.

Bad fan clutches, stuck/balky/wrong thermostats, defective fan
switches - All frequent flyers. Some systems can air-lock, thus
deceiving you into thinking they're topped off,when they're not...

Murphy rules.
--=20
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