Chain slipping off when using front derailleur

NORTHERNER33

From time to time I find that my chain comes off when I suddenly change from the big wheel onto the 39". In particular this happens when I unexpectedly encounter a hill. Is this likely to be a front mech issue?

skyd0g

Yes. Adjust.

Cable tension and/or limit screw.
http://www.parktool.com/repair/readhowto.asp?id=75

...also, try to change gear when the chain isn't under major tension.