Flash help - Shape tweening

Hello all,

I was wondering if anyone could help me with an issue I am having in flash. I made a shape using the pen tool and made it a mask. Then I inserted key frames and did a few shape tweening. It works fine for a while and then suddenly freaks out in a couple of frames then goes back to normal and I can't figure out why. I have the file if anyone wants to take a look, I can email or if there is any way to attach it here I can. Any help wouyld be appreciated. Thanks!