radiator upgrade

What year, what model, what engine, what (if any) mods?

What happened, what were the conditions, what did you observe?

Reading minds isn't a Forum function.

Unless you see a leak, that diagnosis is suspect. If no leaks are evident, it's NOT the radiator.

First thing to check is if the fans are running properly (low and high). There was a recall on '06 cooling fans that failed.

Second is to check fluid level and bleed the system. Air is the system will cause the "boil then stop then boil again after a few minutes" issue. This is the most likely cause of the issue. (although how he saw this without x-ray vision is beyond me). If he was talking about what he saw in the the coolant reservoir, it is definitely low fluid level and air in the system.

Third is to replace the thermostat (they do fail over time). It can also cause the intermittent "boil" issue...though far less likely.

Last is to replace the water pump. It can cause all of these issues if it is pumping intermittently...but last on the list of possibilities.