Locking up

Hello I just purchased a new 2.0 Ghz 20" iMac at a the beginning of the month. Occasionally (about 6 times total) my machine will be locked up in screensaver. In other words the screen saver has been running and when I come back in the machine is locked up. I ran the tech tool app and all items pass. I have made no hardware changes other than adding an external firewire 400 drive and it locked up before adding the drive. Is there a log I can look at to see what I can see? Any help is greatly appreciated. BTW is locks up at random times. Sometimes it will be week other times like today within 2 hours. Thanks

Is there a log I can look at to see what I can see?

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There are a couple things you can use. Console and System Profiler. Both are in the Applications/Utilities folder.