Engine Overheating

I have a 1999 Chevy Tahoe. I recently had a check gage light come on as I was driving. I noticed the temperature had reached approx. 240 degrees. I turned on the heater and the temperature went back to the normal 160 range. When I got home I checked the fluid level, which was about a gallon low. I replaced the lost fluid as required. I noticed the heater hose connector line from the intake manifold with a hairline crack and when I went to tighten it, it snapped off. I replaced the connector line, changed out the themostat (195 degree), and added the lost coolant. After running the engine and heater for a short time I observed the temperature hit approx. 200 degrees before the thermostat opened up and returned back to normal. Now, every time I initially run the engine, the temperature runs up to approx. 200 degrees before it returns back to approx. 160. Fluid level is good. What could be the problem?

5.3l vortec overheats I replace water pump, etc sensor, thermostat, intAke and valve cover gaskets and it still over heats why