temp sensor may be bad telling you it's hotter than it is...
Bubbles in the lines or block keeping it from cooling properly...
Or thermostat could be stuck...

I was a mechanic for a GM Dealership for 9 years and this was what I found. If this has the 3.1 or 3.8 v-6 it could probably be the intake gasket.  The intake gaskets on these engines were made of plastic and would detereorate over time due to the heating and cooling cycle of the engine.  These were tricky to diagnose because sometimes when they leaked you would not see any coolant leaks because it would leak back into the engine.  Does the coolant jar always appear to be low?  Does the coolant appear to be dark and dirty after changing coolant?  

The V-6 is terrible about air pockets. It usually takes a lot of time and patience to get them out.

no it isn't low usually and no never dark or dirty. yes, it is a 3.1.

I totaly agree with jesse its the intakes for sure it very common on those engines. when you replace them make sure to use a vacuum coolant filler to eliminate air in the system.
oh yea if its not leaking externaly you can pull the spark plugs and see if they  have a white film burnt on any of them this is coolant being burnt

there is no way to accurately diagnose a vehicle without having it in front of you stop guessing and hoping, take it
to a certified gm dealer, not your corner street mechanic.

your thermostat could be the problem,also agree with intake gasket hav replaced them to .start with the basic first.