on the most basic of terms you have a problem with water flow or air flow (or both), or your radiator is inadequately sized.
so maybe it's a little bit of all three converging into a perfect nexus of overheating.
and what's overheating? stock gauge is pegged up hot? aftermarket gauge is reading 210? it's boiling over and puking its guts out?
so to circle it back: radiator 1st, is it sufficient? pump, does it provide adequate flow? fan, does it move enough air?
then you have all the secondary items: thermostatic clutch, shroud, distance of fan to radiator, opening for the radiator, thermostat, pulley selection, tune. but here's the deal, none, and i mean none of that matters if you have a crap radiator in there.