I have a 89 xj w/ 4.0L. I put in new thermostat 3 times, 2 waterpumps,radiator and heater control valve. It overheats after 30 min of idle or if i drive a few miles. it seems the thermostat never opens,cooland is circulated everywhere but i know head gasket good,and components are good,even tested thermostat and it opened at right temp, just not on truck any ideas??