Temperature Gauge Is Inconsistent, Cycles and/or Is Erratic

1. Is cycle normal?

Gauge Reaction:

^ Normal gauge reaction to thermostat cycle.

Gauge Reaction:

^ Normal gauge reaction at stop after heavy use.
^ Hot water buildup is normal at stop after heavy use.

2. Is coolant in radiator low (air will build up in the cooling system and cause the thermostat to open late)?
^ Fill system, vent trapped air and check for leaks.

3. Is a cylinder head gasket leak allowing exhaust gas to enter the cooling system (exhaust gas building up in the cooling system will cause the thermostat to open late)?
^ Test for leaking cylinder head gasket leak with a commercially available Block Leak Tester.
^ Check for coolant in engine oil.
^ Inspect for white steam from exhaust system.

4. Water pump impeller loose on shaft, loose accessory drive belt.
^ Repair or replace as necessary.

5. Air leak on the suction side of the water pump (allows air to build up in the cooling system causing the thermostat to open late)?
^ Find leak and repair.