[ubuntu] Ubuntu crashes often; which log file should I look in?

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http://ubuntuforums.org – Hi, I'm getting very frequent crashes with Ubuntu 12.04. These come in three flavours. Unity stops functioning correctly (e.g. applications disappear from alt-tab, or application icons disappear from the menubar). I can often recover with "$ unity --replace". This occurs about twice a day. I see a black screen but still have a mouse cursor. I can still move to tty1 and attempt to recover some information, but "$ unity --replace" almost never works. This occurs once every 1–2 days. I get a full system crash. The graphics are frozen on the last-used GUI, and the mouse and (Hardware)