With everything else new, the rad could be it. If it is leaking it is time to rebuild it if possible, or buy a new one.
It is getting pretty hard these days to find a rad shop that will still rebuild a rad, most would rather just sell you a new aluminum w/plastic tanks radiator, the profit margin is greater. Compare your local prices to radiators.com, or radiatorbarn.com. I am not sure about radiator.com, but shipping is free with the radiator barn.
