When you say "goes dead" do you mean the XMD3 shuts off? If so, it could be the remote turn on wire. Even if you have good ground and 12+, a bad remote turn on wire will make the unit shut off as you say.
It could be that the wire is lose in the harness or the harness that plugs directly into your XMD3. Check your manual and fine the color (usually a solid blue) for remote 12+ and check that wire following it from the XMD3 all the way into the FW harness.
Even if you can't fix it, it will show your dealer that you are trying to solve the issue and aren't a dope.