Finally, MS were able to solv the issue with my MOSS site.
It turned out to be an issue with the navigation nodes, where nodes where duplicated when publishing pages expired.
I had some long running sessions with the guys from Microsoft Support in Madrid and Redmond, and at some time, they could see 1800 SPRequest object calls, each consuming approx. 2MB on the server. No wonder the poor SharePoint was brought to its knees. But it wasn't a memory leek, mind you.
If we just set the timeout on IIS to a very large number, we could actually get MOSS to respond within an hour 
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Now, after MS Support was able to reproduce this behaviour, things went extremely fast, and I received a hotfix within 12 hours.
Apparantly the hotfix is somewhat related to the knowledge base article kb932621, so if you experience similar problems, I'd suggest you contact your local MS representative, with this reference in mind.
steinhof