Replaced thermastat but still overheating - 2001 Chevrolet Lumina

OK, you have addressed the first possibility. There are a few more, each progressively more expensive to fix. First, are the radiator electric cooling fans operating? If you find the vehicle runs ok on the highway, but overheats around town in stop-and-go traffic, it could be that the fans are not running. Quick check for the fans - with the ignition on (the engine doesn't need to be running), move the cooling control to the A/C position. If the fans come on, they are good, but you could still have a fan-related cooling problem. If the fans don't work, you have a problem. If the fans worked, the car runs fine at highway speed but not in town, then suspect the coolant sensor that engages the fans. If the fans work and the car overheats in all conditions, then a system blockage is possible. Blockages occur usually one of two ways: Radiator rust/corrosion, and collapsed radiator hoses. Radiator corrosion will require radiator replacement; if it is blocked badly enough to overheat, flushing it is a waste of effort, and could cause more problems with all the **** you circulate into the engine. If the system is low on water (air in system) it can rise to the top radiator hose and cause it to collapse partially or fully, restricting flow. Some engines have water jackets that extend higher than the radiator cap, so they can trap air. These cars have bleeder ports at these locations to remove to trapped air. Check to see if that is the problem.