Oh. Ok. I just Google Searched "how to replace a tamagotchi tama go screen" and "tamagotchi tama go replacement screen". Didn't find any thing about how to replace a Tama-go screen.
P.S: I'm watching a Hamtaro DVD right now! The Hamtaro DVD I'm watching is Hamtaro Vol. 2: Ham-Hams Head...
Sounds like the battery is dying. Try Changing the battery and try again. That should work. Last night I turned on my Gameboy Color and it kept resetting its self. So I changed the batteries and it worked fine.
Take it apart. It may just be dirt. You never know. But since it grows bigger, it is probably a pixel leak. :( I would wear gloves while taking it apart.
Edit: P.S. Nice icon, Khipmunk! I LOVE Hamtaro! :)