Scan line mask

Hi

I’m trying to make a mask with “tv scan lines”. The mask is animated so that the lines multiplies (the lines get denser) until the mask is completely filled and reveals the masked clip underneath.

I’ve tried doing this by tweening and keyframing (and masking inside the mask mc). Problem is that it looks like crap and flash gives me strange visual bugs.

Have anyone tried something similar successfully? Maybe using actionscript? Help me please! I thought it was a common effect but I can’t find any tutorials or sources.