OK, I've learned my lesson I was able to identify the problem bundle (btw, it wasn't MSOffice but ArcGIS). It had two older versions, I deleted these and haven't had the CPU load spike for 12 hours now. I'll modify the bundle according to your suggestion to install the directory as single piece of content.
For reasons I don't quite understand, the SQL query returns only partial GUID (16-17 symbols) when run with dbisqlc, so, in order to identify the problem bundle I used a rather crude method:
..and searched output.txt for matching beginning of GUID
dbisql -c "uid=USERNAME;pwd=PASSWORD" -nogui -d1 -onerror exit "SELECT displayname,zuid from zbundle;" > /tmp/output.txt
TID7005560 has lots of good advice beyond that specific problem, so I see further chance to improve the database performance.