MS Proxy Server

I am the administrator for 4 NT Servers and Oracle Dbase. I have setup a stand alone Internet Server (NT4) installed with MS Proxy 2.0 Server. There is maximum 10 users concurrently surfing the web with www and ftp services granted. Now my problem is in every one hour or less than that Clients are ‘CONNECTION TIMED OUT’ error even the Proxy is connected to ISP. Connection statistics shows no of bytes/frames OUT but bytes/frames IN is inactive. I have tried several changes yet no permanent solution. Is it a problem with my ISP, Modem or any others. Please provide me a prompt solution.