@kiwidude: Yes, you're probably right here. Interesting that some Win 7 x64 show the issue and others don't. This seems to point to something other than calibre.
With 4GB of memory on my machine, (and only 1.35GB in use), a 4K per 5 sec increase in memory is less than 3MB an hour. At this rate my machine will grind to a halt in ... Ah! It won't grind to a halt: it gets switched off regularly.
I won't be trying to track down the problem after all - it's not worth the effort.