I just recently had work down on my radiator and when I got it back the temp gauge doesn't work right. Sometimes it moves and sometimes it doesn't. When it does work it's running smooth at 190, but then it goes back down for a while then up about 100.
Sounds like a wire got nicked. If you have a diagnostic box, check the results. Otherwise go to a service station or dealer to do the analysis. I would first return to the establishment that did the initial work for resolution. If no solution, you know what you need to do!
You know there is a problem, so do not just take their world for it. Have another trusted establishment check the gauge problem. Above all, do not give up, but remain calm and respectful in resolving the problem.