UPDATE:
THe radiator is definitly clogged. I expect I will replace it soon but with nothing to lose I went looking for a product to break up the sludge.
Autoparts stores no longer sell the good stuff but I found a can of Prestone Heavy Duty cleaner (Oxylic Acid) on the back shelf of an ancient local parts shop. Worked pretty well. dislodged a whole bunch of rust colored aluminum oxide and lowered the operating temp about five degrees.
I also got a digital IR thermometer. Temps go from left to right from 180 down to 160 but there is a cold strip of 125 degrees across the bottom.
