Chevrolet: Overheating, heater hoses, overflow tube

Question
Thank you. The problem is that it is boiling out the overflow tube and not leaking out. Sounds like it is not circulating to me
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-----Question-----
My son has a 99 malibue and it constantly overheats. Had it in the shop and have intack manifold gasket replaced to the tune of 700.00 but still overheats. Do not notice any water in the oil and the heater works sometimes and blows cold air other times. Other than that, the car runs fine. What am I overlooking
-----Answer-----
Mark;
The best thing to do is have the cooling system  pressure tested ,it will show any leaks, but check the  cap, hoses, etc. and the thermastat. also if you get heat half the time, check the heater valve on the heater hose if it has one.  the test will show any leaks.

Answer
Mark;
  I would remove the thermostat and try it with it out. also if there is a heater cont. valve in one of the heater hoses that could be bad , if you get no heat or if one hose gets hot and the other warm or cool.  but try it with the thermostat out. if its ok like that get a new thermostat. or if you live in a hot area you can keep it out or get a 160* thermastat.om