Paul: I'm pretty sure the fan on high bypasses the resistor .Try changing the resistor which is in the fan housing.
He may have bad fan motor bearings.
88BEAST What's your engine temp.? When you changed the core are the hoses hooked right ? Hose from water pump should be on the right side faceing the connections.
Check the post's just below your's . By jastec MUST BE BLOCKAGE SOMEWHERE