the cabinet probably needs adjusting, i had the same problem, had 20mb for 2yrs getting full speed then got 2-7mb 1 month ago, it was due to the rollout of 100mb, they needed to change the frequency of the cabinet to fix the problem as the QAM had changed. Phone virgin tech support and they will check your modem, an engineer fixed the cabinet the next day after phoning, getting 18.7mb now
