I have a 1994 jeep Cherokee Sport inline6 4.0L. I have changed the serpintine belt, radiator, secondary radiator fan, radiator hoses. Still leaking coolant from freeze plug on driver side but not enough to loose all the coolant. After filling with coolant I drive it and it overheats after about 3 miles stop and go city driving. Only options left I assume thermostat and water pump. What one do I check 1st?