If you look in the Virgin.net specific forum you will find that this problem is effecting lots of people, so its unlikely to be anything at your end. I am having the exact same issues, BB with virgin is that bad at the moment you might as well go back to dial up. Virgins network seems to halt at peak times, this can vary between being totally disconnected or having fluctuating speeds, I have had both problems.
Dont bother calling there suport line, they dont have a clue about anything that is computer related. The only recommendation I give you is to move ISP.
Virgin.net 2mb
BELKIN F5D7633UK4A
**Lost Souls LAN Parties**