I have similar problem with speeds as low as 200 on a 1.5mb service, also massive packet loss and web pages taking up to a minute to load - or not at all. It has been like this for months but is intermittent, probably happens about 25% of time. An engineer came to me same day (about 2-3 months ago).
I am not hijacking here, my point is that engineer, once he knew there was a problem has not let it go (I am sure this will be the same for you). He has been helpful well above the call of duty. He has tried many things, spending hours and hours and rings me regularly to get updates and has even lent me a cable modem (I have Samsung STB). Although the problem is not yet cured I feel sure it will be eventually and I have full respect for the engineers, they're doing a great job. (
could understand if I was attractive young lady - but I'm an ugly thirty something bloke!!
It has made me think though, if I can get by with slow connection - perhaps I ought to downgrade when it's fixed!!!