yahoo mail access problem

Friends hope all r better. I'm suffering a problem with yahoo mail log in page for a week. I have a private network and that is allowed to brows internet through proxy and IP's r configured dynamic. Now I discover all the websites seem ok without the yahoo mail within very long working period of day. And the case mostly occurred when a specific PC connect with network either dynamic IP or real IP. This seem peculiar to me and to all whom I discussed with.
So now I'm looking forward to you for kind response.