Frankly, I'm ready for a new radiator on the one.
Very possibly the right answer. I've been through that twice now, once with my previous TR3A and then again with the current TR3. Nothing obviously wrong with the radiator either time; in fact this last time I took it to the radiator shop and they pronounced it "fine". (I made the mistake of asking them to do "whatever it needed".) But the third (or was it 4th) time I took it back, I told the manager specifically that I wanted it rodded and boiled. When they went to rod it out, they couldn't get the rods through the tubes! Apparently, none of the tubes were totally blocked, but they all had enough "mud" coating the inside to block the transfer of heat. Anyway, a new core made all my overheating problems disappear.
The first radiator was still the original type core design, where the fins run more or less straight across and the tubes go through holes in the fins. With it, apparently the tubes were no longer in good thermal contact with the fins, so even rodding it out did not show the problem. But again, a new core made all the problems disappear. The difference with it was really dramatic, I went from having the gauge climb slowly at freeway speeds even at 70F to staying cool even at 115F.
Of course there were a lot of other little improvements made along the way, trying to solve the problem, which I didn't bother to undo later. But the new core was clearly the right fix.