[fredjacksonsan]

I'd also consider changing the cap, rotor and wires along with the plugs.

The hot thing is troublesome and although the engine could be done like rhandwor suggests, I'd make sure the cooling system is good...water pump, thermostat, radiator. On an older vehicle that has not been well maintained your radiator can be blocked enough to let it overheat.

There are several threads on overheating XJs in here...and since it runs fine while cold that is where I'd look first. The detonation is when it gets overheated IMO.