Hey everyone, thanks for the input! I will address each item that has been brought up:
1. the overheating problem was pretty much spontaneous, there was nothing done to the car to cause this problem. Although the summer temperatures have probably exacerbated the problem or brought it to light.
2. The heater core is filled with coolant, verified by heat output.
3. I am running a 75 degree thermostat, made by Wahler (OEM)
4. The ignition timing IS correct at idle as well as at high engine speeds, both with & without vacuum.
5. Itís very, very unlikely to be a blockage. The coolant has been changed *every* year for the past twenty years, and the original radiator didn't even have much scale on its internal passages. There has never, ever been even a slightly rusty appearance to the coolant whenever it was changed, therefore it is not a rust or corrosion related problem.
6. Coolant dilution is correct, in fact it still behaves this way when running a lower ratio of coolant/water, which has more effective heat transfer capabilities.
I donít really know what else it could possibly be! It seems unlikely to be a head gasket problem, at least from my experience. Maybe Iím wrong Ė but being a technically oriented person, could someone please explain to me how a (possibly slightly(but no evidence of it)) leaking head gasket could cause an overheating condition like this?
Iíve thought of everything I can, I even checked the camshaft timing marks to see if maybe the chain jumped one tooth Ė this is how desperate I am; I mean the likelihood of that happening is practically Zero!
Thanks Again, and keep those ideas coming!
Still Going Crazy, (although somewhat comforted by the camaraderie).