Have you at any time in the past put tap water into the coolant?

the suggestion RE water could be a good one - and it could be a head
gasket - your car should be good for over 300K miles no problems - the
variation in compression is a bit suspicious

try this - take an old spark plug and modify it so that you can attach an
air pump to it and pressurize one cylinder at a time - just weld an adapter,
or alternatively, take your compression tester apart so you can use it to
pressurize a cylinder.

remove radiator cap.  one at a time, pressurize the cylinder to whatever max
air pressure you have (my pump goes to 160 PSI) - look for bubbles in the
coolant tank.   My guess is that if it is a crack in the head or block you
will see bubbles with one cylinder only, if it is a head gasket you will see
with two.

in either case, mark the ones causing bubbles and then pull the head and
look to see what happened.

It will probably turn out to be the head gasket, but there is an oil cooler
which is cooled by the engine coolant(antifreeze). I am told that there were
problems with leaks between the two systems. As the oil pressure is higher
than the coolant pressure oil gets into the water system and raises the
pressure above normal. Check for signs of oil in the water.

Alec