I had a 126 that overheated also and it started this around 50K miles. I couldn't find a thing wrong with it and did some research and a guy that did rad repair said he had found on those cars the rad plugs up in circle right behind the cooling fan. He had no idea why but he thought it might have to do with it being cooler in that area. I don't know but I replaced the rad and had no troubles again.

But of course you should change the thermostat, cap and 50/50 mix first.