ZyXEL P-660HW Series router problem

Hi,
We changed to post office broadband around a month ago, and we've been experiencing connection problems a lot, where the router will loose connection and then require you to turn the router off and then back on in order for it to recconect. However the internet connection only stays for a small finite amount of time and then it is lost again.
We've replaced the router 3 times and each time has done nothing.
I'm currently linked up with a ADSL cable, and there are two computers using a wireless connection to it.
I've tried countless times to renew my IP address with the windows connection repair tool, but it has made no difference to the problem,
If anyone has had these problems and has solved them it would be greatly appreciated if you could tell me how, as it is looking at the moment like we will have to change broadband user's, which may end up costing us in excess of £100 for cancelling the post office contract.
I feel i should also note that we changed our ISP from Orange to Post office.

Thankyou

Check this forum post out: http://www.the-scream.co.uk/forums/t23364.html . It may help