Have you replaced the sending unit on the engine?
Sending unit failure is just one of those things that happens.
Do you want a mechanical or electric gauge?
Mechanical: normally more accurate, also can be more hassle to install. Routing the capillary tube and keeping it from vibrating is the problem.
Electric: easier to install, (just requires a wire to power when the key is on, a wire between the gauge and sending unit and a wire for the light, this is required for both unles you only drive during day light hours) can be replaced with out having to remove a capillary tube, normally cheaper, accuracy depends on quality you purchase.
Both will require draining a certain amount of coolant, installing a reducer bushing and sending unit, then refill the system.
Both usually come with good instructions.