Macbook: You need to restart your computer...?

I have had my 2.4Ghz white MacBook for eight months now, and from the very beginning it freezes every week or so and pops up the message "You need to restart your computer. Hold down the Power button until it turns off, then press the Power button again." It is displayed in several languages. There doesn't seem to be any pattern of when it happens. I have done some research and know it's a kernel panic and have tried everything that Apple's website recommends, such as re-installing the OS etc. I even took it into a Mac store and had them try to fix it -- they updated everything, but that hasn't solved it either. Does anyone have any suggestions or recommendations?