Hi there,
I will try to answer your questions.
Buying a modem off ebay is illegal as the person selling the modem is selling something that does not belong to them. The modem remains property of Virginmedia.
Any cloned or other modified or sourced modems are illegal as they can 1) allow a user to get free services or 2) disrupt the network or cause problems to other customers.
I am not accusing you of anything as I can see that you want to get your issue sorted and are a genuine customer.
The only thing I can suggest is to continue trying to call Virginmedia and telling them that you need a new modem as yours is faulty.
Tell them that you have tried it on different computers and run all diagnostics, disabled anti-virus, etc, and it has not made a bit of difference.
Hopefully someone on here who works for Virginmedia can see you are having issues and contact you.
I know it must be fustrating as sometimes they will accuse you that it is your equipment when you fully are aware that everything is fine your end. Hopefully you will finally get through to someone at Virginmedia that will take your issues seriously.
to the forums by the way.