You can confirm the presence of combustion gases by having a shop place their emissions analyzer sniffer above the filler neck, if "HC" readings go up, bingo!
Not all head gaskets leak coolant into the oil, some leak into the intake passages. I'm surprised this engine isn't running hot....if you cannot maintain system pressure in the radiator, the boiling point of the coolant goes way up, placing a vacuum on water does just the opposite, lowers the boiling point.
That's why you keep a pressure cap on the radiator. 15-16 psi is normal, should raise the boiling point to 230-240 degrees.something like that.
You haven't noticed any unusual smoke/steam coming out from behind when you accelerate have you?
Hope it's an easy fix, I'd be looking for cracks in between the combustion chambers or maybe it's just a gasket issue.
Hth,
Ben
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