Outlook 2010 prompts for a password regularly

We are in the process of upgrading from Exchange 2003 to 2010. We are still using Exchange 2003 as our server but all of our PCs are running Outlook 2010. We have users in our Chicago office that connect to our Exchange server over a T1 who regularly get a password prompt in the lower right in the Outlook window. Originally, they connected with cached mode. But when these users began reporting these password prompts, I changed their connection to use Outlook Anywhere thinking that this connection method would be more tolerable but it hasn't helped. It appears that our T1 is running fine but since upgrading them from Outlook 2003 to 2010, Outlook seems to be very sensitive. I've read several posts and one of the more common threads was to increase the timeout setting by doing the following:

Hit the file tab, and then account settings, and then double click to open the email account. Now click "more settings" and then the advanced tab. You will see a slider to increase your timeout.

However, when I go into this dialog box, I do not see this slider. Is it because we are using cached mode? In any case, does anyone have any suggestions for resolving this? Thanks.