That's an unacceptable level of service! I think you might well have grounds for terminating the contract early on that basis.
I use VirginMedia's cable broadband (used to be ntl) and never had problems of that sort. I've only had one problem with them in about 6 years. I lost my connection, phoned India who said it was the modem and sent me another one ... which was wrongly labelled with the code number ... they sent me another one ... still didn't work ... eventually they sent an engineer, who found that the problem was with the splitter outside the house, which I suspected all along. Took me a month to get back online. Other than that, it's been great.
The lesson is - don't believe what they tell you!