The front derailleur has two screws on it... one controls how far in towards the frame the derailleur can travel, the other controls how far the FD can travel in the other direction. See this page (
http://www.parktool.com/repair/readhowto.asp?id=75) for instructions on finding which screw does which.
Basically, it sounds like the derailleur is traveling too far in one direction and it's pushing the chain clear off the last chainring.