Not knowing anything about your application, I'll take a guess at this one. I would suspect electrical noise as the most likely culprit. Make sure you are properly grounded and sheilded, and make sure you do not have excesive wire length. You can also try putting a small (pF) capacitor on your input to act as a simple filter on the noise.
Sheilded twisted pair wire can often work miracles in these situations.
Hope this helps.
