This is a good thread. DS5 is similar. He can instantly do some things that would take most adults longer, but can make careless mistakes. Sometimes, when it's a problem with a reading portion and an illustration, I've caught him just assuming he knows what they're asking by looking at the illustration, and giving an instant answer (a good part of the time that snap answer would be correct if his assumptions were correct, but I can't reward him for that).
He has gotten a lot better at being methodical through practice, but is not perfect by a long shot. I don't worry about it because he's only five, and I think that he's far ahead of the game on practicing being methodical as well as everything else. But I definitely think that being methodical is going to take practice for almost anyone, and that it may take extra practice for a kid who does mental math quickly and is easily bored.
