Thanks, that's what I needed to know (and suspected).

The problem hasn't recurred in the last 2 days, and only seems to crop up with a frequency of about once or twice per week, where CPU usage spikes to about 18-20% for a short period before apparently fixing itself back to it's usual level of well below 2%

I've put in logging mechanism now so that if a page takes longer than 0.02 seconds to render it records which domain and what action of the system was being called, hopefully when the next spike occurs that will be enough information to single out the cause.

Once again thanks for the second opinion, it was useful!