How can I fade in and fade out a movie clip using Set Interval?

Complementary info : im using the photogallery code I found on this website
I bet it’s really quite simple, the movie’s got to fade in with _alpha += 0.5, stay onscreen for a short time and then fade out. All this works with an onEnterframe, something I’m not completely comfortable with…
I’d love to send the fla but it’s way too heavy.
Any helpfull souls around?