2006 Macbook black 2,1 The bottom row of keys, zxcvbnm, works sometimes (like just now) and not at all other times. I'm trying to understand if I've got a mechanical problem or a software problem. Intermittent problems are hard. It's an old machine but has no problems, in fact the top case was replaced a couple of years ago while the cracked plastic was still eligible for replacemnt by Apple. Any suggestion appreciated.