I haven't used Testors DullCote for over ten years, and even then it was out of the spray can, but I always had consistency problems with it. One can would be dead flat, and the next would be semi-gloss or satiny (yes, I checked the labels to be sure I was using DullCote!). Never any problems with clouding or melting decals, just problems getting the flatness I wanted. I have never tried DullCote through an airbrush, so maybe it's more consistent.
What I use now, however, is Gunze Sangyo's Aqeous Clear Flat. That stuf is dead-as-doornails flaaaaaat every single time I use it, out of a spray can or out of my airbrush. I can't imagine using anything else (although I still want to try Future mixed with a flat base).
If you haven't tried Gunze, you really should give it a shot.