Check the engine oil for any signs of foaming. Check the radiator coolant for any signs of oil contamination. Make sure you bleed the cooling sytem properly to get all the air out. Make sure the fans are coming on. With the radiator cap off, observe the coolant to see if there are indications of flow when the engine is running (could be a broken shaft on the water pump). Are there any signs of coolant loss?
sometimes what seems a complex problem can be something very simple to fix, have you replaced the radiator cap or had it pressure tested? a radiator cap not holding pressure will cause over-heating in modern cars with all the demands made on the engine.