It's probably on the payment gateway side. Does this gateway have any web based tool for you to view failed transactions? I first start to see if it is hitting the payment gateway in some sort of failed transactions. Maybe that will give you the answer there. If it's sending unknown code it should be hitting, if it is hitting the gateway it isnt storefront issue unless you modified code then I would look at the processorder call.
SQL 2008, Visual Studio 2010
Source: C#
Version: AspDotNetStorefront ML 8.0.1.1/8.0.1.1
Framework: .Net 4.0 (Running)