It's all becoming clearer now...

The times I have had trouble with my device when I have had to remove the battery to reboot, it is quite possible that I have left the back off while it rebooted. Then it would have rebooted and not detected the internal card, and run slowly.

Giving up on the device and seeing if it worked in the morning DID actually work, because I would then put the rear cover back on, and the next morning it would work....

So it had nothing to do with wiggling the card around in the socket - it was all about the back and the magnet!

But why would it need the cover to be on? Why have a magnet to check?

This N800 is like a totally unsuitable lover. I adore it but it drives me mad with rage sometimes...