I was investigating high resource usage on one of my hosting packages. The max allowed I/O was being consistently reached, usually very late at night/early in the morning. This corrolated with very high CPU usage as well at the same time. Processes and RAM usage seemed to be very normal at those times.
I contacted the hosting company to help me figure out what's causing these massive spikes, they basically told me they don't know :S
They also mentioned they saw the TTFB was very high and this is most likely due to lots of resources needed to build the page. They recommended disabling all plugins and reactivate them one by one to see if any plugin causes the high TTFB.
Not a big fan of doing this on a live site. Has anyone had problems with this as well?