Your power levels are quite normal for your connection, the may be a local issue or the modem may be acting up in ways we cannnot see from power levels.
You could try calling Tech Support to see if the are any issues in your area or posting on the newsgroups.
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Tech Support is open 24/7 on the numbers below.
For help with your TV or Phone and for Broadband Technical Support:
Call Product Support/Faults on 151 option 3 from your Virgin Media Phone. It's absolutely free.
Or call 0845 454 1111 from any other phone line.
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Or
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Tech Support via Newsgroups.
virginmedia.support.broadband.cable is the one that you want to use and here is the set up link
http://www.virginmedia.com/customers...setting-up.php
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