Remove upper and lower radiator hose, hold your hand over low radiator outlet, fill with water, remove your hand, water should dump out like a toilet flush.
Next even though you replaced the fan clutch check it. Your running cool at lower rpms so low speed operation of the fan is probably working. At higher rpm on the highway the fan may be turning to slow.
High flow water pumps sometimes cause cooling problems due to coolant moving to fast thru the radiator. ie not enough time for heat transfer.
