overheating

Looks like you have done just about all you can do except.... I'll just throw a couple of similiar problems that I've had with my 93 Maxima. I eventually had to replace the radiator because of a flow restriction in the very center of the radiator. This is the area that the electric fans cover and there was no coolant getting to this area causing overheating. Another cause could a temp coolant sensor malfunction. This sensor signals the cooling fans to come on at a specified coolant temperature. If this is malfunctioning the engine can overheat at times. An easy way to test this is with the engine at operating temperature and running check for even heat with your hands when the fans come one. If it is hot on the outer edges of the fans and cool in the center then you have the same problem as I did. Even thou you had a complete flush of the system there still may be some debris restricting flow. Good Luck!