unstable Transaction Scope and MSDTC error: Communication with the underlying transaction manager has failed

I used transaction scope in my web service and encountered exception of  "Communication with the underlying transaction manager has failed." The weird thing about this is that the error occurs very rarely. According to our web service log, only two out of  300+ calls led to the error. I am pulling my hair out here. Please help. Thank you very much.

 The code in the web service is as following,

using (TransactionScope scope = new TransactionScope())

{

customer.Add();

Foreach (CustomerCase case in Customer.cases)

     case.Add();

scope.Complete();

}

The  IIS and Database server are on seperate windows 2003 servers. Both servers are on the same LAN without firewall in between. The MSDTC security setting on both IIS and DB server is as following:

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