This appears to be a common Windows 2003 IIS problem. Usually you have to do the following:

• In IIS Manager, expand the local computer
• Expand Application Pools
• Right-click the application pool
• Click Properties
• Flip to the Performance tab
• Under Web garden, in the Maximum number of worker processes box, type 1
• Click Ok

 

Turns out this setting got changed when we were trying to pin down a different perfromance issue a couple of weeks back. Set it back to 1 and everything appears to be back to normal.

 

We actually use the session object quite a bit though, so I'm a little surprised we didn't see other symptoms ... we had a couple of peopel comment on being logged out a bit faster than normal, but that was it!

 

Anyway, thanks again for your help, much appreciated.

 

Matt