Sorry, was not home for some time and somehow missed this thread before
Even on a 920, if the device otherwise works fine, the bootloader might be the key!
Use my resue BL, or the ones you already have in your collection to get the 5.5120 working on the 920 and try again to connect to the PC.
If still no luck:
Open the thing, take it all apart and check the connector very closely, if you have with a cheap USB Mirkoscope or a good maginifying glass.
I'm almost certain you will find bend/lost spring connectors inside the connection system or broken solder connections.