Thats interesting and might relate to my problem. I DID change a freeze plug towards the rear recently and yes, there was a bunch of waxy/grease looking crud behind it. I used a radiator flush on it about 3 times with no cessation of the over-heating problem however.
20 years ago I flushed the radiator and it all came out black...subsequent flushes have barely been discolored.
Now things are back to normal...not to say your suggested ball of crud wasn't to blame, just that if so, it has shifted favorably...
