If you've swapped the 2 boxes around, and the problem has followed the box, that's a pretty good bit of fault-finding, looks like you've done most of the things I would suggest and your next step will be to call Virgin Media and report the fault.
150 from a VM phone, or 0345 4541111 from any other phone. If they can't resolve the fault over the phone, they should be arranging an engineer visit or replacement box.
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