If you're trying to extract a lot of info out of relatively short interview, consider the approach I sketch out in On Interviewing Candidates. It's a technique that dives in from a higher level, exposes thinking, and is less sensitive to false negatives if a candidate doesn't happen to see the trick in a trick questions. It also gets you to a point where you can say "now, show me code".

The risk of relying soley on programming problems is that you'll get false negatives if you catch someone on a bad day.