1997 jetta temperature gauge

Here's the problem: when it is cold out, my temperature gauge on my dash sometimes doesn't get to where it's supposed to be and often fluctuates. I was wondering if this is a coolant sensor or some other sensor. If it is a sensor, where is it located and how do I replace them? Possibly some thermostat problem?

Also it stays at the optimal temperature during the summer, but when it gets cold out that's when the problem starts.

Any help would be appreciated. Thanks.