Where does the plane of the front ring hit the cassette? If you have your bike in a stand you can just turn your front wheel to look through the spokes and sight across the chainwheel to the cassette. Should be about the middle of the stack, maybe one cog either way at most.
And a couple other questions........
- Which side is it falling off?
- You aren't shifting gears while in those high torque situations where the chain falls off are you?