few things to check
- chain length. while in middle chainring and large cog, where is the derailleur jockey positioned? Should be pointing between 4-6 o'clock. If jockey is positioned at say... 7-9 o'clock.. this is too loose and will result in the chain flying off.
- What gear are you using when the chain pops off?
- Many new outboard bearing cranks have horrible chainlines and are prone for this type of behavior. So... what crankset are you using?
- Lastly, buy one of these to prevent the chain from going all the way down to the bottom bracket. http://www.ronniering.com