Hi Allan, I do indeed have the latest BIOS, and all the other latest drivers in fact. It certainly appears to be a hardware issue rather than a software/firmware issue. I have tried two different Microsoft products which both have the same behaviour: Microsoft Wireless Mobile Mouse 4000, and the new Microsoft All-In-One Multimedia Keyboard. Both use a nano transceiver. I have not tried using a nano transceiver linked with a non-microsoft product, nor a standard 'large' style receiver.
I too thought that it might have something to do with the way the chassis is used as the wireless antenna, but I don't understand why using a USB extension cable rectifies the issue even when I then place the extension cable with the transceiver directly on the chassis.