why is the temperature gage showing high?

The temperature gauge can show high for the following reasons: 1) faulty gauge 2) faulty sending unit (both of those concerns not likely, but possible) 3) Air pocket in the cooling system. If you have replaced components, and an air pocket still exists, the engine can still get hot and boil over. 4) Faulty thermostat 5) Cooling fan failure 6) Air flow restriction - possibly between condenser and radiator 7) water pump not circulating coolant (unusual) 8) internal engine coolant leak, like with cylinder head gasket failure. Certainly there could be other reasons, but those are the more common ones. Testing is required to find out what is going on.

More details about when or under what conditions the temperature gauge is "showing high" will be very helpful... Idling at stop light .. Going upgrade .. Highway speeds .. All the time... Also is the cabin heater output normal > good and warm ..