There was someone else on here with a similar issue, and in their case it turned out to be a new (dual-core I think) radiator that solved the problem.
If there isn't air in the system, and the cap is holding proper pressure, I would think it sounds like a stuck thermostat (maybe not fully closed, or just doesn't want to open fully) or the water pump - I'd assume and hope not since it was replaced, but that would be my next guess.
If it isn't running lean or something, and the ignition-timing is correct or close to it, the only thing else I can think of is maybe running a flush product to remove the build-up from the rad. etc.