Ok, after 2 weeks of High mysql cpu usage I have to ask, how can i keep this down, and how do i find out which user is doing it?

During peek hours mysql uses 50-200% Cpu usage. This in turn is causing server loads of 0.70 up to 2.25 on a dual core

There are not that many databases on the server, an has only started in the past few weeks, i've checked the current users who use mysql and they have not upgraded, changed, or added any new scripts or databases.

This is causing the server to fail abotu every 4 days causing a reboot by the DC. The downtime's are hardly anything to notice as reboots are fast but its irritating.