I get this problem too, and I thought it was the keyboard, yet I think they are all built to only handle a certain amount of bytes at a time.
8 bits every character. It does it if you press about 3 at once?... possibly 4? Thats 32bits it can send, or 4bytes at a time.
Though, you know some schools? They use modified keyboards with pictures, and you push many keys down to make 1 selection. They dont have problems do they?