Attempted to read or write protected memory and Search portal

Hi,

I am experiencing a problem where randomly we lose the search dialog box on the Sharepoint Search portal of a customer intranet, the error logged is :

SearchBoxEx::CreateChildControls:System.TypeInitializationException: The type initializer for 'Microsoft.SharePoint.Portal.WebControls.LocStringIdLookupTable' threw an exception. --->System.AccessViolationException: Attempted to read or write protected memory.

Load control template file /_controltemplates/AddContentSourceControl.ascx failed: The type initializer for 'Microsoft.SharePoint.Portal.WebControls.LocStringIdLookupTable' threw an exception.           bb7f60e7-9fb7-432f-98c0-1b74b1c13fe6

10/01/2012 02:36:44.34 w3wp.exe (0x07D8)                           0x1940  SharePoint Server Search                 Query                                    dn2p                High        Exception in SearchBoxEx::CreateChildControls:System.TypeInitializationException: The type initializer for 'Microsoft.SharePoint.Portal.WebControls.LocStringIdLookupTable' threw an exception.

This is fixed recycling the Application Pool, but temporarily, because the search box dissapears again.

What could be causing this?

Many Thanks