has been with us since the days of analogue modems, and is a constant source of debate. ADSL downstream speeds - for that's what you're talking about - will vary considerably, and often from day to day, depending on the contention rate. The more people that are online at any one time, via a particular gateway, the greater the chance of a speed drop.
Nowadays, if you are in an area of high-density living (flats, housing estates, etc.) the chances are that you'll be contending with another 40 or so people for access to a limited bandwidth, and you'll see slow downstream rates.
It might be that,it might be a Tiscali hardware problem, or it might be a combination of both those things.