Too much noise to hold the 1.4Mbps sync so it resynched lower to get the noise margin back up
I would suggest going to another ISP if Virgin won't fix it. This behaviour isn't normal and isn't acceptable regardless of line length.
Could you please start a thread in
http://forums.thinkbroadband.com/general.html explaining the big swings in noise margin and connection speed? No offense to CF but that is more geared towards DSL and someone there may have a contact within BT that can assist.
Cheers and sorry I couldn't be more help Stuart.