2001 Nissan Pathfinder radiator

I think I need to replace the radiator on my '01 pathfinder. In winter, my engine runs cool, and in summer, under load or when the A/C is on and at idle, then the engine will run hot. I am pretty sure there is a blockage in the rad somewhere. Any tips or advice on the best way to replace this?

I am a do-it-yourself kind of guy, and would appreciate any help. Thanks