Oh, i missed the part about the new rad. So, heater core cant be it, if there is a water valve that bypasses the core, and its closed, could back up some coolant flow. Airlock? If it was vacuum filled its highly unlikely. Water pump with poor flow at idle. Belt installed incorrectly, maybe turning bass-ackwards.
All that tested and or verified. I did have him refill it with new coolant, again, He said his machine does it while under pressure to keep air out, so I’m guessing that means vacuum but I’ll ask.