magpienja,
You get what you pay for in this life.
The less you pay, the more you get contention, traffic shaping, rate limiting and all sorts of other dark wizardry the ISPs use to stuff as many punters onto the same pipes.
If you do a lot of downloading, streaming or similar, then you'll be hit harder and sooner by their technology under the auspices of their "fair usage policy".
The fact of the matter is that on a residential contract, you're not going to get any SLA/SLG provisions that you get on some (not all) business contracts. Therefore large fluctuations in the performance of your line are to be expected and are par for the course.
Shame they don't tell you that little tidbit on their shiny TV adverts isn't it....
P.S. Try
this template for your complaint letter (NTL acquired by Virgin, so still applicable).
