Assuming you are connected to a Virgin hub a wired connection is obviously the best option, you could try using wifi to see if the problem persists, if as you say everything has been working ok then maybe a factory reset could be tried because vigin have been rolling out some software updates. Just select the option to keep your recordings as it's not really a factory reset you can keep your recordings and series links but will have to sign in to netflix etc again. Also some settings will default like dolby audio, standby timer, and match frame rate, I think thats it.
If the reset doesn't make any difference I would definitely follow japitts suggestions with the ethernet cable and ports, or try wifi, if it works with wifi then it probably is a cable or port fault. Version 4.31 is not one of the newer versions being rolled out slowly, I am still on 4.31.
Sorry to hear you've encountered this issue with your 360 box's connection. If the helpful advice offered above doesn't clear this up for you please get back to me via PM and I'll be able to take a look into it from here.