Vista - internet connection grinds to a halt-lights are on but no-one is h

I have tried to resolve this problem on and off for some weeks before posting
here!

I have two Dell Laptops, one at XP and one at Vista Home Premium. Each
laptop has an internal Conexant modem and a USB external Trust ISDN modem.
The XP connects fine to the Internet over either modem but the Vista machine
does not.
I have upgraded the ISDN modem driver on the Vista machine to Vista 32-bit
version.
Both modems connect the Vista laptop fine for a period of 2-20 minutes but
then the connection stops. The modem appears to be fine, the ISDN line is
fine, the Network and Sharing Centre diagram looks fine but the number of
bytes being transferred freezes. The ISDN Status is "Connected" and similar
for the Conexant modem.
When I run 'Diagnose the Problem' I get 'Cannot connect with DNS Server' and
a bunch of things to try, none of which work.

All help appreciated.

On 8-Nov-2007, =?Utf-8?B?cMOhaWQ=?= <pid@xxxxxx> wrote:
Try static IP addressing. Google for it, there are many useful guides.
In essence right click on the network, and you should get to properties.
Properties should have the option to untick "Let Windows Manage
This Connection"
Then you should have a set of boxes to set
Your/the machines IP address
It's Gateway Address (its way to the outside world)
Subnet Mask (Commonly 255.255.0.0)
Main and alternate DNS server (Often the same
as the Gateway address)