Any chance that a program is running that wants an internet connection? File sharing, web sharing, a mail, time or weather program that periodically tries to update itself? Try opening the Remote Access control panel under Setup and pressing the Options button. Select the Protocol tab (PPP). If Connect automatically when starting TCP/IP applications is checked, uncheck it. Go to the TCP/IP control panel in advanced mode and press the Options button there. Make sure that Load only when needed is checked. If still no luck, try turning off all non-OS extensions (OS x.xx All). If that works, search for an extension conflict. If it does not work, I do not know. Good luck.