Intermittent empty form pop ups

A problem has started occurring more often recently, with some new users to our Nav 4.1 Employee Portal / WSS 2.0 intranet. When they go to fill out a Purchase Quote and open up a pop up, to enter a value from a list into the form, sometimes that pop up is blank. This can change every few seconds, on a page that has not been refreshed. In 18 months of using the EP we haven’t encountered this problem before. Has anyone else? The server itself hasn’t been rebooted for a while, so we’re going to try that tonight, but I am not sure if maybe there is something else at work here. The NAS gets restarted every weeknight to allow the database backup to occur.

Just to be exact, the ‘pop up’ is set up, in the EP Group template, as a text field that is ‘Editable (Indirect)’ and looks up to a list that is defined elsewhere in the Group template.

Is this problem on the web page or is it on the web page? If on the web page have you seen what’s the characteristics of the users who have the problem compared to those who does not have the problem? Are they running the same OS, browser etc? I have seen similar errors (not with Navision) where it always was due to problems with the client.