Problems accessing a secure site from certain ISPs

I am not sure if this is an IIS problem but I thought this would be
the best place to start. We have a secure intranet site setup on
Windows 2000 SP4 IIS5 and all hotfixes have been applied. Clients are
all XP with IE6 and all hotfixes. The site works fine internally and
works fine from most of our employees home computers. However, the
employees who use Comcast cable or our MCI dialer get a "page cannot
be displayed" error when connecting. I can access the site with no
problems using my RoadRunner connection at home.

Here is the strange part. The users with the problem are able to
access the site using the IP address, so I thought this was a DNS
problem. But when they do an NSLOOKUP using their ISP's DNS servers,
it returns the correct address. They are also able to ping the site
by name and it pings the correct IP address. But when typing the name
of the site into their browser, there is a pause and eventually they
get "page cannot be displayed". Some people actually see a
certificate warning message popup before they get the error, so I know
that they are getting to the server.

I have tried all the steps in Q813444 and it has not resolved the
problem. Anyone seen this before or have any suggestions on where to
begin troubleshooting? I have contacted the ISPs in question, but
have gotten nowhere with their tech support people.