Reply to Comment
Another (techy) update
For those interested (TECH-ALERT!):
Turns out the eAccelerator PHP extension was not very amused by the new x64 kernel and/or SMP setup. Not sure exactly why, but it caused a lot of random crashes if the web-server module. This resulted in empty pages being rendered AND cached. So once a crash occurred for a particular URL the result (empty page) would get cached and delivered each time the browser would request that particular page.
So, to fix that I could try updating / recompiling eAccelerator or give APC a try - which I wanted to do for quite some time. I chose the latter one.