Replaced radiator

let me count the ways- long story short, try bleeding the cooling system, there might be air in there- or MAYBE your overheating problem wasn't the radiator after all? sorry, gotta go now-

Any time I've bought a used radiator I take it to a radiator shop to pressure test it and check for good flow. It's not worth going through all that hassle for a bum radiator. But then again radiators are so cheap new, it's not really worth taking a chance on an old one.

I would say air lock