My Tiscali 150kbps Broadband service stops working about twice a week!?

I have Tiscali's 150kbps Broadband Internet Service. They call it "Broadbandx3 - Unlimited"

I have had it for a few months now and I have found that about twice a week I can't connect. The problem lasts for many hours and then suddenly I can connect. I have tried to contact Tiscali's Technical Help, but I can never get through and speak to anyone (BE WARNED IF YOU'RE CONSIDERING TISCALI - THE TECHNICAL HELP AVAILABLE IS VERY VERY POOR)

What can I do myself? Can anyone suggest anything that I could try? I am using the USB modem which they provided. It does work, because I do use it sucessfully most of the time.