what is causing the overheating in the 2002 rendezvous?

not sure if this helps you or not since your saying you flushed it, was this done by a professional or yourself? I was experiencing overheating. I replaced the thermostat, overheating persisted. I took it to buick and had a diagnostic done for 120. they told me I had air in the system, they bled it, and the overheating subsided. I asked them how air got into the system, since the overheating happened before I opened the cooling system to install new thermostat. That they shrugged shoulders about, but I guess it did not matter since bleeding air out of the cooling system solved the issue

Check the thermostat and or replace it.