Insist on speaking to 2nd line or the team manager then 2nd line, they will/have to put you thru if you insist and refuse to get off the phone.
You could also insist on an engineer visit. If the engineer comes out, plugs his laptop in [after rebooting the modem] and all works fine, there will be a charge though, so be warned!
I work for Virgin media and if your connection works fine with USB but not Ethernet [im assuming you rebooted the modem after swapping over as this MUST be done due to the cable modems being..... not the best] then it does indeed sounds like a replacement modem is needed!
Also im not too sure how much your paying but as for everything in life you get what you pay for!
I work for the business sector and we provide cable modems as a business package, yup its a touch more than the residential product but you get 1000 times better support imho...
Anyway, insist and kick up a stink....
Good luck