session data

I'm having a problem with session data. I have a login setup which holds the user ID and password in the session data once the user has initially logged in. When the user goes to a new page or accesses a pop up window the users session data is validated against a list of IDs and passwords held on the system rather than forcing them to log in each time. As far as I can work out this is fairly standard stuff for this sort of process. It works fine in Firefox but is very inconsistent when used in IE 7. On occasions when a new page is accessed the users is forced to login again. The reason for this appears to be that the variables in which the ID and password are stored don't exist. I have a whole load of trace code which gives me various information and session ID is apparently being picked up correctly. The weird thing is that if you reload the page it will then work correctly. I have tried adding session_write_close as I thought the data was not being written out correctly during the initial login. This code has been developed in eclipse using an Apache web serve and works just fine in this environment. I am trying to deploy it to an IIS serve to which I only have limited access and cant debug it in this environment.