Low coolant

For the last 6 years, the coolant reservoir has stayed below minimum. If I add coolant, it spits it out and returns to the low level it prefers. I have had that looked at and gotten no answers. No warning lights or overheating has occurred. There are no leaks. 2 weeks ago I added 50/50 mix before a long trip. It spit it out and returned to its preferred level as always. There were no issues on this trip. 2 days ago, while driving on the highway the temperature gauge shot up to almost red, stayed there about 3 miles, then returned to normal just below half way. While driving in town, it ran slightly higher than normal, but normal at highway speeds. When idling at stop lights, the gauge crept up but returned to the middle range while driving again. At no time did the red indicator light for high temp come on, but the gauge constantly fluctuates and crept up while idling, running higher than normal while at low speeds. It ran normal temp on highway. The reservoir is now nearly empty, slightly lower than it usually runs, so I assume whatever issue prevented it from staying above minimum level has worsened and is now causing temperature issues within the system. I just don't know what the cause has been since the beginning.