I got a car and an AC adapter for my mini, one of the cheap ones in a bundle from a vendor on Amazon. They would tend to disconnect or go on and off. It turns out that the HP mini plug has a little more exposed metal and a narrower front - only a millimeter or two, but enough to prevent the universal connector (i.e. the kind you get at radio shack) from seating properly.
It turns out that if you remove some of the plastic around the base of the plug so the metal has a few extra millimeters of clearance to go all the way in and seat properly, it works 100%. I would be careful about torquing the plug since there is less support plastic, but it is no longer intermittent.