2001 Jeep Cherokee overheating

This is what I got for you. I have an 2001 Jeep Cherokee
with a 4.0l, 6cyl. Auto. I was having internal cooling lose. I pulled the cylinder head, took it to a machine shop. They found a crack, so I bought a new head. I've put it all back together with brand new head gasket, exhaust/intake gasket, valve cover gasket, thermastat and gasket, spark plugs and of course new fluids. Okay this is were it gets tricky. It will start, sounds great, heats up fine, until I take it for a drive then it will start to overheat. So I replaced the thermastat again and have now replaced the water pump and radiator cap. What else can it be besides maybe the radiator itself. Because I'm thinking thats the next step. But after that I'm completely stumped. Any idea?