Since you seem so sure it's not a heat issue, then logically it's got something to do with your PSU. But you say it's new (550W), hmmmm.. Could it be your AC input voltage dropping below 10% of your normal supply? Try an AVR on your AC Line. Another thing -- take out the whole mobo and peripherals and run them outside the casing. If it works well, your system casing needs more ventilation. But then, that would be a heat issue - which yous say is not.
