Haha I'm not familiar with companies that work in computer science. I'm more of a hands on-trades type of guy. Point is, perhaps the interviews aren't successful because you aren't the skillful candidate that they are looking for? I imagine a slight speech impediment is not as big of a deal in computer science as it would be in radio broadcasting? You don't need to speak much for your work yeah? It would be a good move to search out particular skills that employers are looking for by asking them, beforehand being better than during or after the interview.