I have a 99 eldorado with only 85k miles. I recently replaced the coolant, thermostat, and radiator. Now when I drive her she stays at normal operating temperature. But I have noticed when I get stuck in traffic or when I am driving in hilly areas that the temperature will fluctuate between normal to about 3/4 on the temp gauge. Any insight as to what to look for? I have a hard time believing that the temperature of the coolant can change this rapidly? Also, Can this happen if you overfill the coolant? Thanks for your time!

 

Well, I can't believe that you haven't gotten any replies! I'm gonna tell you the little that I know. Change the coolant and learn to understand the new response you get from your gage. There's nothing wrong with the new coolant.
Just keep a close eye on it for awhile and make sure that it's topped off. You and the car get to learn about each other.

 

It's pretty normal but as Bob said watch it