Would it be possible that your stem might be too short? I've found that when I use a shorter stem, my hands start to go numb but with a little bit longer one, I'm fine.

Proper saddle adjustment will help too, in case that might be an issue.

Wrenchscience.com and Coloradocyclist.com have good fit guidelines but I've found that the best thing for me was to take my bike to the LBS and ask them to help me, which made a world of difference. Its amazing how a mm here and a mm there can really help or harm!

Cheers,
Kristina