Making a scrolling shooter, can't figure out glitch!

[AndyGH]

Okay, so I've got a pretty simple scrolling shooter going on, I started working on it in a CS3 class I was taking and at the very end of the course I ran into this problem and haven't been able to figure it out on my own! I know exactly what the error is, but I have absolutely no idea what's causing it or why, and it happens completely randomly. Sometimes I can play the game for 10 minutes and it'll work perfectly, sometimes I can play it for 10 seconds and then it will glitch and freak out. I'd like to try and explain it here more, but I feel as if it would be much easier for you to just run the game and see it for yourself.

I've attached the .fla, I hope someone here can figure out what's going on! I apologize if my code is rather messy, as it should be obvious I'm still a Flash and AS3 noob and my formating habits probably aren't up to standard.