I don't have much experience with Guinea Pigs, but I just wanted to add a story. My good friend has 3 guinea pigs that she rescued from the local pound, all on seperate occasions. All 3 were bad biters. She doesn't know the age of any of them, so who knows how long they had been without human contact. All 3 have come along way, but they all still bite on occasion. So don't give up, you can break them of the habit (or at least for the most part), just keep handling her.

Oh, also welcome to pet talk!