Read my lips: Virgin Media are so awful that I'm leaving them and I encourage you to do likewise.
Reasons they're awful? Let me give you a list. To start with, I didn't much pay attention when they announced that they were going to start charging by the minute for telephone support calls. After all, I'm competent to configure my own broadband router; I don't need my hand holding, right? Well, I've changed my mind.
It appears highly likely that Virgin are probing the equipment you attach to your cable modem and dropping packets destined for broadband routers.