If you recenlty purchased the Mac and it's under warranty, take it to an Apple Store to have it looked at. It seems like your Mac might be overheating. I know there were some issues with the first generation of MacBooks and MacBook Pros (the ones with the Core Duo processor) in regards to overheating. I haven't heard anything like this with the newer Core 2 Duo MacBooks and MacBook Pros that were released towards the end of 2006.