Exchange 2007 CAS Watson error precedes SMTP unresponsive by about 5 hours

This is on a Windows 2008 server running Exchange 2007 HTS and CAS.  This problem happens about twice a week where we get the following 2 errors, then about 5-6 hours later SMTP becomes unresponsive and we need to manually restart the Microsoft Transport Service.

Event Type:      Error
Event Source:      MSExchangeTransport
Event Category:      (10)
Event ID:      10003
Date:            12/17/2009
Time:            6:54:06 PM
User:            N/A
Computer:      GN-CAS.gemini.edu
Description:
The transport process failed during message processing with the following call stack: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Exchange.Protocols.Smtp.SmtpInServer.HandleConnection(Socket connection)
   at Microsoft.Exchange.Protocols.Smtp.SmtpInComponent.HandleConnection(Socket connection)
   at Microsoft.Exchange.Transport.Main.Program.HandleConnection(Socket clientConnection)
   at Microsoft.Exchange.ProcessManager.ControlObject.CommandReceived(Int32 size)
   at Microsoft.Exchange.ProcessManager.ControlObject.ReadMessageCompleteInternal(IAsyncResult asyncResult)
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)

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Event Type:      Error
Event Source:      MSExchange Common
Event Category:      (1)
Event ID:      4999
Date:            12/17/2009
Time:            6:54:06 PM
User:            N/A
Computer:      GN-CAS.gemini.edu
Description:
Watson report about to be sent to dw20.exe for process id: 3672, with parameters: E12, c-RTL-AMD64, 08.02.0213.000, edgetransport, M.E.Transport, M.E.P.S.SmtpInServer.HandleConnection, System.NullReferenceException, 69f8, 08.02.0213.000.  ErrorReportingEnabled: False