My 2005 Mazda 3 Hatchback recently started randomly near over heating. Sometimes the gauge goes almost to H very quickly. Other times it gets 3/4 way and goes back down. When the heater is on, it will blow really hot then cold then hot again. AC doesn't work well at idle but does while driving. I pulled over today, opened the reservoir it hissed and filled with hot fluid. Temp gauge went back to midway and no more over heating. It will likely do the same thing when I drive home from work again. Any ideas?

I have the radiator with 1 fan and the fan works.

 

any engine codes? Thermostat possibly? If it's stuck closed intermittently and not letting the coolant circulate that could do it.

 

It turns out, I had a coolant leak. Two faulty hoses. I'm glad it wasn't serious!!