well one thing that is often looked is the pressure cap on the expansion bottle. when the car reaches operating temperature the cooling system creates its own pressure of about 1 bar. Because 99 percent of car rely on convection currents within the system as well as rediation and conduction. If the pressure cap seal is broken it may not create its pressure and cause overheating i had the same problem after replacing everything else.
dean the mechanic, May 2010