Install munin, it's a great resource tracker that creates graphs for cpu, ram, disk, mysql etc usage over time see
http://library.linode.com/server-monitoring/munin
Install htop it's a prettier version of top, just run htop from the command line, it can be sorted by cpu, ram etc so if you sort by cpu it will show you the heaviest processes at the top.
Does this happen at regular(ish) intervals or is it random?
You should check your web logs to see if you're getting a flood of traffic when it crashes, also check your syslog to see if there's any warnings or errors in there.
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