I had a similar problem in my commute car recently. It was slow to heat up, then would suddenly spike to a high temp, then return to normal. It turned out that I had a pinhole leak in a small water hose that would leak under pressure and vibration but close when the engine cooled. The symptoms with the gauge were caused by being extremely low on water in the radiator. I didn't figure it out until went through a sequence where I topped off the radiator and watched the temp gauge carefully for about a week. Most of the week the temps were normal, then it started acting up again due to low water. I finally found the leak by driving the car for a while, then parking it in my garage with the engine running and going under the hood to wiggle all of the water hoses. One of them squirted when I wiggled it, but didn't leak if it was stationary. Replaced the offending hose, everything has been fine for a week, and the water level hasn't changed either.
Marty