I realise I'm not adding anything, or helping (sorry), but my N900 started doing this last week too. I hadn't installed any software or upgraded any. The strange thing is, it fixed itself a few days ago and is working normally.

I still have no idea what the problem was. I don't remember for certain, but it might have been helped by rebooting, though I didn't reboot to fix this problem (the phone was getting sluggish).