[mtajpa]

Not an expert but sounds like the beginning of a blown head gasket. Pull the plugs and look for reddish deposits and or even maybe water droplets on the plugs? Do this after it has been idling a while. When you have the plugs all out turn it over with the lanyard pulled to see if water is squirting out the plug holes. Small amounts of water in the combustion chamber will also clean carbon from the plugs.