I'm getting regular emails to notify me that the CPU usage is over a certain threshold at 90%. Often it goes over 100% and then eventually everything goes down and I have to restart the server.

There are only 3-4 sites running on this box with very few visitors; There's one Wordpress site and the rest are custom php sites developed by me, nothing complex. I'm sure the Linode can handle it fine. It runs on Debian.

Can anyone tell me how to start debugging this? I'm a developer with very few linux skills but can follow instructions

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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