I battled a cooling problem a while back. Flushed the system multiple times to no avail. Then I had the radiator rodded out and everything was fine for a day, but the problem quickly returned. Running out of ideas, I installed a Tefba cooling system filter, flushed the system again and cleaned the radiator and no problem since.
It turned out there was (for lack of a better term) crud in the system. It looked like dirt/rust particles that presumably were slowly breaking loose from walls of the coolant passages, lodging in the radiator and inhibiting coolant flow. The filter did trap a bunch of particles that I saved in a jar to keep a running tab of what was in there.
I believe I have the same crud, but mine seemed to stay in the engine block and rot the frost plugs out in a short time.(three or four seasons and only a thousand miles)
