Slow Net

Hi
I am facing a problem form past few days in acessing my web site hosted on the server on my lan.
i have NT 4.0 server with two ip set one for local lan one as public ip. I have MS proxy server 2 running on other server which acts as a gatway for my lan.i have our devlopment sites hosted on these servers.
Now the problem is if i want to open any web site it opening with good speed but when i try to open my own site from local server giving public IP it’s taking long time while it takes less time opening with local ip. This problem has started from past few days before this everything was working fine. Please help.