Well, it just gets more and more interesting...
After re-installing, the Kobo would fail to connect, although the computer gave the "bong" noise indicating somethign attached. Investigating with the device manager, I found a "usb hard drive" was disabled (code 22). Attempting to enable it was successful and it was the Kobo. Now, it appears to stay connected.
So, I think the culprit was a excessive number of USB memory sticks attached. I had always assumed that the memory sticks were innocuous and that with USB, there was some physical address limit to the number of devices; a number so high as to be impossible to reach for practical purposes.
But, another lesson learned; USB sticks can interfere with kobo by some mysterious mechanism.