I purchased a 98 legacy outback (2.5 DOHC) that had a blown headgasket.
Block and head appeared flat (as well as I could measure with straightedge and feeler gauge). I replace headgaskets, seals, timing belt etc.

Have had a periodic overheating issue since that is driving me nuts!!

Have tested the coolant system for exhaust gases 3 times (with vapor pump test fluid kit) and all 3 tests show head gaskets OK.

Have replaced several thermostats, then drilled a few small holes in the thermostat. Never will overheat with even just a single 1/8" hole drilled thru thermostat (just to side of valve area in thermostat), but just not quite @ operating temp, so interior heating is not great.

Replaced the water pump!

Radiator has been blown out, and radiator cooling fans cycle as they should.

With a new OEM thermostat, runs up to temp. After a while of driving, you hear a squeal (almost like the alternator belt would squeal) then loose interior heating, and then the engine will quickly overheat!

Been working on cars for almost 40 years, but this REALLY has me stumped!

Any suggestions would be appreciated!