We have an ASP.NET 3.5 application that has been in production for over a year. Our last release was a couple of months ago. We use CSS for styling and application of background images to divs and such. The server is Windows 2003 with IIS.

Suddenly, this week, we have had reports from some users that the page seems to hang up while loading. The status bar was showing the name of a background image used in the page main area (assigned in CSS). At our office, some of us could recreate the problem, while others could not. IE6 and Firefox do not seem to be affected, only IE7/8. Running Fiddler on an affected machine and trying to see what was happening with the requests seemed to make the problem go away (while running through Fiddler, it returned when not). Hitting Refresh on a hung load often made the page load just fine.

I checked the background image, and even replaced it with an archived copy. No joy. We re-deployed the app from our production source. No Joy. We restarted IIS and eventually rebooted the whole server. There are no unusual entries in the event logs, the app logs or the IIS logs. Finally, I removed the image entirely and re-styled the page not to use a background image. That solved the problem at least for now.

However, we have reports of other images "hanging." The images are PNGs, but I have heard some rumors that sometimes a GIF hangs, but I have no screenshot to confirm.

This just started happening "out of the blue." There have been no releases or updates applied to the server recently. We even checked updates on clients to see if a recent Windows Update might have caused this on the client, but there was nothing updated within the last couple of weeks.

If you have any information about this problem, I would love to hear from you. I would also greatly appreciate any recommendations on additional diagnostics we can try.