undefined pressure on 1993 Honda Civic

Is there anything other than a blown head gasket that would cause one of the four spark plug boots to 'pop up'?

Engine runs great
No excessive heat
225k miles with outstanding maintenance

radiator leaking between core and plastic tank
(waiting to order online replacement)

Asked by for the 1993 Honda Civic
Yes, the oil seals wear out inside the valve cover and this causes the plug boots to get really oily until the boots pop up. Also, you may have a loose plug which is leaking compression in to the spark plug well and forcing the boot out. I would replace the valve cover gasket and plug seals with factory parts from the dealer. They are e more expensive but they work really, really great. The wires are most likely pretty oily so they need replacing as well. I would use NGK or dealer wires. If you don't replace the oily wires, there will be misfires as the spark jumps though the wet, oily boots.
Local counterman at Cut Rate Auto said my problem is most likely a bad gasket in the valve cover that is allowing some engine gases into void. He said it is a common Honda problem.