It's really hard to say from just that. What peripherals do you have plugged in? Do you have third party RAM? Did you notice this happening after making any changes to your system? If you open up Activity Monitor in the Utilities folder, do you notice anything using up a lot of CPU cycles or RAM? Make sure you're looking at All Processes, and not just My Processes. Sorry for all the questions, but they may help us get to the bottom of this.