When was the last time that you re-torqued the cylinder head? Should be done yearly! Your symptoms are characteristic of a blown head gasket. Do a compression check with a gauge when cold and again after the engine comes up to operating temperature.
When the engine is cold the head bolt/studs are thermally shortened. When at operating temperature there is elongation (stretch) and you lose the ''clamping pressure" of the head. Proper torque applied gives preload tension to the head bolts.
Second possibility is a slightly sticking exhaust valve stem. Just get inside and manually push the exhaust valve up and down a few times to be sure that the valve stem isnt binding. If it is, spray on "PB Blaster" and work it until its free .... then look for a pin hole in the wet exhaust manifold that is leaking back into the combustion chamber when you shut down the engine. If there has been water back-flowing into the combustion chamber, get some "Marvel Mystery Oil", pour some into the sparkplug hole and soak the piston rings free. Use 3/4 regular motor oil and 1/4 MMO when you fill the crankcase thereafter.