The impossible to solve overheating problem

Does the upper hose get hot? If not, you have air in the system, a bad thermostat ( I know you checked it, but...) or the water pump simply isn't working properly. I personally would suspect the water pump, as I have had issues like this in the past. You should be able to see coolant flowing rapidly in the system by running the motor with the rad cap off. If the problem is trapped air, there are several methods of "burping" a cooling system if need be.
Start with a cold engine. Remove the radiator cap and fill it to the recommended level with a 50/50 mixture of antifreeze and water. Make sure any auxiliary tanks are also at the proper cold fill level. With the radiator cap off, start the engine. Make sure your heater is set to maximum heat. You want 100% circulation of the coolant. Let the engine operate long enough to warm up enough to reduce the idle speed to a normal idle. Let it idle until it's at normal operating temperature, keeping an eye on the temperature gauge. Overheating can cause additional problems, obviously. Once the thermostat begins to open, don't rev the engine, even slightly, as this can force hot coolant out of the open radiator.
You might notice some air bubbling in the coolant out of the open radiator during warm up. This bubbling is normal if you have air trapped in your system, and is caused by the air escaping as it reaches the area of the open radiator cap. Unless the bubbling is excessive, continue to let the engine run at idle for a few minutes once it reaches normal operating temperature, then shut it down. Allow the engine and cooling system to cool off, preferably overnight. Then check your coolant level again. If it dropped, that means you've displaced some of the air in your system. Refill to proper levels, and repeat the process. Do this until the level doesn't drop any longer, then replace your radiator cap.