I just experienced the same thing - eager to see the replies here ��

 

Update on this: issue solved. It was the water pump that was failing. Replaced that, but then didn't burp the engine properly and the expansion tank was boiling over like it was a certain blown gasket. Really scared the bajeejas out of me! I've never seen an air bubble affect the cooling system so badly before (and I'm 52, with plenty of wrenching).

Anyways, after a while to ponder, and reflect that I'd better start over rather than begin head gasket calculations, and then the system settled perfectly, and to this day is running fine, with its new water pump.

A note that the PO had removed the bleeding T-connector at the top of the engine, and replaced it with a short copper straight-pipe. Not a good idea. Replace with a better-designed bleeder, if necessary, but not a straight cut.

 

Thanks for the update, always nice to post such things to help others resolve their issues.