Recently I have had issues with the coolant in my car. At first the temp gauge would move to just above high and immediately move back to normal. It would do this every 5mins. I started by just replacing the thermostat and coolant (no flush). It ran well for about a week of hard driving with no issues. I then took it on a short road trip. On the way back the temp gauge went to beyond high and the cap overflowed. I was able to drain and replace the coolant again and drive the 3hours back home. Once home I attempted to flush the coolant once I got home. I followed the post here and got to the point of actually flushing. The tank will boil over every 2 mins when trying to flush. I made sure when filling the tank that it stayed between the lines. I am not sure where to go from here. Is there air bubbles, the water pump, corrosion, clog, gremlins.....I bought a new water pump but I am not sure that is the issue. There were no signs of it going bad (sounds, leaking,). Please help.