Propellant can problems, and "splattering" not misting problem

Ok first, Since I can't afford a compressor I'm using a 6oz Testors propeelant can thaat came with my Aztek a270.  As the informational video says, after 2 minutes or so the can gets really cold (as in getting frost on it) and the paint stops coming out.  I'm not sure if the pressure is dropping, because when I stick my hand in front of it, the airflow feels roughly the same.  But if I let it warm up, it's alright again.  Second question.  I know its a fairly cheap, single action airbrush, but I stillt hink Im doing something wrong to get these results.  When I spray on paper with black water based paint (thinned to the consitency of milk, like the video said), i doesnt give an even "mist" of paint that fades int the paper.  It sort of sputters.  You can see little specks of paint, and it doesnt look very smooth at all.  In some places, theres too much on the paint and it drips, and some places theres almost none, and all over, its that splattering problem.  Ive tried all 3 of the tip sizes too.  Even when I spray with water instead of paint it doesnt it a little, although not as badly, but its hard to see.  If you dont understand the problem I can post a scan of the paper.  Im sure some of this is just since Im so new at airbrushes, but what am I doing wrong?  Thanks guys, Ian