Temperature gauge not working properly

[SK]

The temperature gauge of my 63 Eldorado does not work properly.

It goes slowly all the way up and stays there, while the true temperature is around 155-160 F (water in the radiator and/or the top of the thermostat housing).

Where should I start to solve the problem? The gauge itself, the temperature sensor, or the wiring? Or something else? The sensor is a new (aftermarket part) and the thermostat worked fine when I tested it in a saucepan. 

Many thanks

-Samu

[Jason Edge]

As Chris noted, most new temp sending units (at least the ones from China) are mis-calibrated and give a false hi reading. I have a good used sending unit that gives a resistance in the correct range I have seen for good sending units over the years for $20. I only have the one for now and the demand has been growing. I can ship to US destinations for $10 for a total of $30.  If you want it, shoot me an email to [e-mail].  

[Chris Codd]

I was just talking about this with Jason the other day as my car is doing the same thing. The new temperature sensors can be junk straight out the box. I believe Jason might have an extra used original one. I just ordered one from him yesterday.