Post what modem you have and your power levels from your config pages of your modem. The should be a sticker on the bottom saying what model.
If using an Ambit modem please click here
http://root:root@192.168.100.1/CmOpConfig.asp
If using any other modem click here
http://192.168.100.1/
What I want you to post is the Downstream power levels including the SNR and the Upstream power levels for me thanks.
DO NOT POST YOUR MAC ADDRESS OR SERIAL NUMBER
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You need to call Tech Support which is open 24/7 on 151 from your Virgin Media Phone.
It's absolutely free.
Or call 0845 454 1111 from any other phone line.