Needing direction

I have a wireless network in my house that is served up by a Linksys
WRT54G router. I live in a rural area without DSL or cable, so the
internet connection to my house is served up by an ISDN line and a
Netopia R3100 series router which feeds the WRT54G.

Our two wireless enabled laptops see the wireless network OK, and I can
ping/telnet/ftp/tracert out to destinations on the web such as
www.google.com, www.microsoft.com, etc. However, when I open a browser
session (either Internet Explorer or Firefox) to www.google.com, the
browser churns away for a while and then throws up a page not found
error. I try this for www.microsoft.com, and any other common web
address and I get the same result.

While the browser is trying to materialize the page, I do a netstat and
I see an entry with Google's IP address like so:

64.233.179.104:http ESTABLISHED

However, the browser never presents the web page.

Other oddities:

I can update my Ad-Aware software from the web, but Spy-bot will not
update. Also, I can try for 30-40 minutes to get out with the browser
with no luck, then leave my PC for a day or so (no reboot), and try
again and the browser works, leave for a couple of hours, and the
browser doesn't work. When the browser is down, my Cisco VPN client
does not work, but if the browser works, I can VPN to my office.

Sometimes, I *THINK* I can kickstart the browser by going in and
changing the subnet mask on the ISDN router to any value in the range
255.x.x.x and that may work for a while, but then again, the browser
stops working at some later point.

I have tried using IE browsers on two wireless laptops: a Dell running
Win XP Pro, and an older Compaq Presario running XP Home. Both have
the same problem

I have also tried this on my home brew desktop running XP Pro with IE
and Firefox, and tried it with the machine both physically plugged into
the wireless router and/or the Netopia ISDN router, but the results are
the same.

I thought maybe my browser or port may have been hijacked, so, I have
run virus scans, ad-aware scans, spy-bot scans, cwshredder, you name
it, but nothing seems to work.

I have been running the ISDN/WRT54G scenario for about 3 years now, but
the Netopia has ony been in the mix for about the last 6 months, and
this problem started about 2 months ago.

Has anybody had a similar situation where their browser just stopped
working, but every other port connection to the internet still seemed
to work?