with my stock radio the antena went up when the radio was on and went down when I turned it off or switched it to cd or tape... same with my new stereo. If you take the stereo out you will c that there is a power remote cable for the antenna on the back. this cable should only have power when the radio stations are used... this is y your antenna will go up only when the radio is turned on and will lower when you switch to cd or turn it off.
As far as working some times, could be that the casing is not on tight and the gears are shifting. could also be that the teeth on the belt for the antenna are worn or even the gears are worn inside the casing. Could also be a bad ground or a loose connection in the back of the stero to your remote antena wire.
Hope that helps.