1999 Cadillac SLS overheating

Engine Cooling problem
1999 Cadillac SLS V8 Automatic 135000 miles

my car keeps overheating I have replace the overflow tank, overflow tank cap, radiator, and the thermostate twice. When I am driving for about 15min the gage in the car starts to move up then when I press on the gas to speed it up sometimes the gages moves back to the middle. I work about 22 miles from home and when I get up the next morning sometimes there is water in the overflow tank and sometimes I need to add, there is no white smoke or the is no foam in my oil, what should I do now. I live in Arizona and it gets very hot in the summer.