Engine temp problems can be a number of things and sometimes its multiple things. The temp guage sender is located at the rear of engine where the intake manifold meets the cyl head and right beside the valve cover. The sender screws in and has a single wire connector thats purple with a yellow stripe.
If it seems like the engine is running erratic temps you should check the coolant level & condition and flush the cooling system if neccesary. Test and/or replace the thermostat, make sure the cooling fan clutch is good and functioning correctly, make sure the A/C condensor and radiator are clean and free of debris.