How often? Phones, nowadays are little computers, and freeze up occassionally, just like PC's. If it does it all the time take it back to your service provider.
I am also facing same problem actually you need to delete some data from phone or memory card sometime this type of problem arise because of huge data store in it. It is better to delete some data and then check again. Its also fine if you format your Memory card. I think after this your mobile freeze problem is solve.