I have recently put a new thermostat and coolant flush on engine. Before that the temp gauge was moving all over the place from normal to cold to hot back to normal back to cold back to normal and the heat in the truck was not blowing heat all the time it would go from hot to cold. I then did a coolant flush and thermostat change. Truck ran great after for a week. I then noticed that the temp gauge with go up between hot and the middle but then come right down to normal. I also noticed when It would do that the overflow tank cap would be blown off and coolant would be blown out. Also when coolant is building up in reservoir and the radiator is lower. Also there is gurgling in reservior when u shut truck off.... Answers???