There are lots of things it could be; you might have to much downloaded onto your computer and it can't handle it or your disc might not be clean.
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A Nintendo DS game may keep freezing because the gamecard is dirty or worn out.
There are many reasons why a Nintendo DS game may keep freezing, but the most common issue is dust in the game card.
It is your computer/laptop that is making Moshi Monsters freeze.
All you have got to do is keep trying!
Programming glitch.