What was done to the car prior to your overheating problem? Did you just have it flushed? Anything replaced?
1- Did you re-fill your heater core also when the coolant was replaced? To avoid air bubbles in the core, you should disconnect one of the heater hoses and fill the core using a funnel attached to the hose, reconnect the hose afterwards.
2 - What kind of anti-freexe are you using? Use only MBZ or Prestone for aluminum.
3 - I know you probably don't want to hear this but head gasket might be a possibility eventhough compression is good. It might be letting very little pressure escape once the engine is hot which may not be detected by a compression check. If that is the case it would be better to just bite the bullet and have the heads pulled. Don't forget to have the timing chain, rollers, etc replaced as good insurance.
Good luck and let us know how it goes.
1987 Jaguar XJ6