I think you may have a dodgy router. I had a netgear router which worked fine for around 2-3 years then started doing as you described. I bought a new Linksys draft N spec router (300mbit sec) and the problem was solved. You only need to give them the MAC address of the Modem/set top box if you change this.

You can buy a cable broadband modem from www.scan.co.uk for around £25 iirc. Also do you have any software firewalls on the pc's?