Intermittent Umbraco Exception

This is an intermittent exception thrown intermittently in Umbraco 3.0.4 running on IIS 6

When umbraco user logs in and sometimes selects a document to edit, he gets this ...

[NullReferenceException: Object reference not set to an instance of an object.]
umbraco.controls.ContentControl.addControlNew(Property p, TabPage tp, String Caption) +242
umbraco.controls.ContentControl..ctor(Content c, publishModes CanPublish, String Id) +832
umbraco.cms.presentation.editContent.OnInit(EventArgs e) +442
System.Web.UI.Control.InitRecursive(Control namingContainer) +321
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +692

      

        
        
        
      

    

Once an error has occurred, it will persist for every user logging into umbraco until we restart the application pool (it runs its own in it), so some global state must be corrupted, but it might work for several days without a problem, but then it may take several reboots in one day.

If I debug the application it always works and if I run debugvw on the server in a broken state it won't give me any more information. (except HTTP link)

There are many deadlock messages around the world, such as this one , but none of them represent any specific way to reproduce.

I know this is a long shot, but has anyone here experienced and resolved this issue?

The update is unfortunately not an option right now.

Thanks everyone.