I have the same problem as you. I have rang virgin 4 times!! my connection is still 2mb, its downloading at 270kbs max. It starts off at a high speed then just drops straight away. Virgin are sending an engineer round next week.
If this problem isnt resolved im going to get rid of virgin. Im fed up of the rubbish service they provide.
---------- Post added at 14:20 ---------- Previous post was at 14:19 ----------
p.s this site is good for checking your speed
http://www.thinkbroadband.com/download.html