Pete,
It sounds like a possible flow problem.
As I said before the waterpump did help me.
On the radiator, even though "The radiator is relatively new and had been cleaned during my rebuild a year or so ago. " I have found that a couple of things can influence it's ability to function properly. ( On my sons car we had/have a new 3-core [2-core was standard], after 30min of running we found that deposits in the engine had come free and were causing a 25% blockage) This is primarily due in some cases to the engine (or radiator in your case) sitting dry for a period of time. I bought a filter (I'll post the name when I get home) that fits in the upper hose. I was amazed at what the filter caught after multiple flushes with the usual chemical flush kits. (Chunks of rust, sand from the original casting, calcium flakes all of which would have surely clogged the rad again. Other checks are a malfunctioning t-stat and/or collapsing lower hose (this is where a vaccum is created by the water pump to pull coolant out of the rad.- the higher the RPM's the more vaccum that is created) timing, and then you get into the tests I mentioned prevviously.
Another place to check is are you "burping" all of the air out of the engine? If you have a plug on the intake remove it while filling and this will help get the air out (replace it when coolant starts coming out
).
take care we'll get it......I'm gonna try putting a spring in the lower hose of my sons car (havent' tried that one yet).
Mike