The only thing I can think of that will increase compression is if something (carbon deposits, oil,coolant,fuel) is taking up space in the combustion chamber causing the air to be compressed into a smaller space. If there was coolant in the cylinder from a bad head gasket it could increase the pressure. Is the coolant low? Can you see anything in the cylinder? It does seem like an unusual symptom for a BHG although the other symptoms match. You could try pressurizing the cooling system with the #5 spark plug out and listen carefully for hissing or gurgling. Good luck.
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