where some orders are going through successfully but some are not, i would suspect a problem with the billing and shipping address iinput.
this part of the error:
Address Verification Response Code: D
does not make sense. D is not a valid response for the Address Verification Service, see this page for valid values and what they mean:
I suspect a bad billing or shipping address or a bad CVV code is being entered. But without being able to reproduce the problem, it is next to impossible to tell what the cause may be.
you may want to add Full request and Full response bindings to the failure email so you can see exactly what is being sent to the gateway when the failure happens