I still reckon there's something wrong with their DNS setup if you can click to some www addresses and not others.
The only places your machine can find the IP address for a www you type into it, is either from a cache it's storing, or from the DNS server your ISP told it about.
I've had the problem a few times when my ISP has changed DNS servers, or when a site I use has changed its IP address and the change hasn't percolated through the system yet.
(That's where the TTL (Time To Live) for the IP address matters - if it's set to a long time, the DNS server probably won't refresh till the TTL is up. If it's too short, that can cause other problems.)
If there are a few addresses that you use a lot, then post 'em here and I'll look up an IP address that you can "fix" into your hosts file - that'll make it use those IPs rather than the DNS results. It's a kludge, but it works (I've done it a few times in the past).