Hi Florian,
Is there some timeout or maintenance script running
only all five
minutes that could cause this?
Sounds like a caching related problem. See
http://www.mediawiki.org/wiki/Manual:Cache for further information.
You might also have a look at
http://www.mediawiki.org/wiki/Manual:Job_queue and
http://www.mediawiki.org/wiki/Manual:$wgJobRunRate which regard to periodically run job in
MediaWiki.
hth
Frank