Initial user authorization successsful, then fails over time

[stratcat900]

I'm using a Quad Core G5, OSX 10.4.6, connnected to a large Windows network. There are instances where my authorization on the network fails after an initial successful connection. Two examples:

Example 1: Entourage connnecting with our Exchange server. When the application is launched, it connects successfully to send and receive mail and update calendars. At some point during the synch process, an error message is generated: "Logon failure:unknown user name or bad password" It gives you the opttion to try and re-enter the info, but the corrrect info is not accepted. If the application is closed and then re-opened, again it makes an initial successful connection and then authorization fails.

Example 2: The company employs web-filtering software, and I'm given specific acccess permissions for a variety of site types that are generally blocked. Those permissions are granted based on username and password. On my Mac, I can navigate to a page on one of these sites (initial autthorization), but cannot navigate further in the site (authorization fails for subsequent loads) I'm able to navigate freely on my Windows machine using same username and password

In both examples, the initial user authorization process is successful (keychain info is correct), but subaseqent authorizaitonss fail. Any help greatly appreciated...IT here refuses to suppport the Mac OS, so I'm on my own in finding a resolution

[stratcat900]

BTW...I'm able to log onto and stay logged onto shared windows servers on the network.