It took some fiddling about and in spite of my methodical approach I can't say exactly what the problem or the solution was. My theory is that my previous DC-In board had problems, and somehow made my previous adapter act a little wacky. Replacing the DC-In board and replacing the adapter seems to of done the trick.
I'd like to tell more detail for anyone else finding themselves in my shoes but I'm going to give it a few days of trying it in different situations before I make any definite statements. I just didn't want the suggestions and comments to run rampant with ideas when I seem to have found a solution. Thank you so much to everyone for their invaluable input! I'll update with more info on the problem in the coming days!