Well, sorry to bore you with this, but since tomster.org is the only Plone instance experiencing these issues, it is also the only instance I can use to debug them...

To recap: whenever this Plone instance is active, CPU-usage on the host is erratic and constantly jumps between almost zero and 99%. After optimizing some feed performance issues on Friday and then sticking a squid instance in front of the site for good measure and then leaving everything on for a couple of hours I got this:

The first two peaks were due to lengthy compile runs on the system (while tomster.org was off). Then from Tuesday to Wednesday I turned off the static maintenance page and reroutet all tomster.org traffic to the plone instance. Wednesday to Friday I switched it on and off again experimenting with CacheFu settings until I finally installed squid on Friday night. Saturday morning, before heading off to an offline weekend in Stolzenhagen I switched everything off...

My next approach is to deactivate the tomster.org site product and see what happens if I serve the site using the native Plone skin. It's sunday, 10pm, I think I can risk leaving it on for a couple of hours over night... We'll see, what the morning brings...