Overheating

After driving the car for a while the temperature gauge needle starts climbing all the way up to high. If I am going down the freeway it cools it back down but when I start going very slow in the neighborhood it is back up.
a compression check was done and the pressure is fine. Does not always heat up though, sometimes I can go all night long without it heating up at all. We are changing out the thermostat now to see if that happens to be the problem.