There's a possibility that it's a power levels issue, so to eliminate that first can you post your Upstream and Downstream data ... enter 192.168.0.1 in your browser to enter hub admin ... don't sign in, just click on Router Status.
If you go to http://192.168.0.1/ (or http://192.168.100.1/ if you are in modem mode) to get the VM hub GUI. Don't login but instead click on Router Status which is either a button in the top right of the screen or a text link at the bottom of the screen depending on which hub you have. Click on the Downstream, Upstream and Network Log sections and copy & paste the figures from those screens and post them here, don't worry about the formatting or you can post screenshots if that is easier. Be careful to edit out or obscure any reference to your WAN IP or MAC address in the log to protect your privacy.
The community can have a look for you and see if anything in the figures looks wrong and advise accordingly. Or you can wait for the VM staff to pick up this thread (can take 7-10 days) and run checks on your line.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.
Apologies for the late reply but we waited for a few days to check performance. I have to say things appear to have settled down considerably, probably through a combination of checks & changes. So, I won't make any more changes for the time being but I am intrigued when you say you managed to check our system - could you tell me what you checked & how, please?