Engine running hot

Are the fans turning on? If they are not, the engine will not overheat at highway speeds when there's sufficient natural air flow.

Sorry for the delay.

Next, feel if the entire radiator is getting hot. If there's cool spots on it, the flushing didn't completely clear the blockage. That blockage is a common result of GM's use of the red Dex-Cool antifreeze. We call that stuff "Dex-Mud". To verify that, run the heater on one of the higher speeds. That will act like a small radiator and reduce the overheating. Also see if the cooling fins on the radiator crumble off easily. It's a little too soon to have that problem but if they do, they won't effectively dissipate heat to the air.

The next thing to consider is a leaking head gasket. Combustion gas leaking into the cooling system will pool under the thermostat and prevent it from opening. Thermostats must be hit with hot liquid to open. Your mechanic can do a "sniffer" test to check for a leaking head gasket. He will draw air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present that liquid will turn bright yellow. That test is a little harder to do on GM vehicles that have no separate radiator cap, but it can still be done.