bokes

First off, I have a Surly XCheck which is not made for a triple crank, yet i want really low gearing, so I put a triple on anyway and i'm only using the small and middle rings. With that i'm using a double front derailleur. (a triple derailleur wouldn't work, cage was too long and would bang into the chain stay)

I've had 2 bike shops adjust the derailleur the best they could. Yet when shifting to the smaller ring, it falls off once every 20 shifts (shifting from the small ring to middle works flawlessly). I'm guessing this is not normal, because i don't see Tour de France racers getting off their bikes several times to place the chain back on the rings!

Actually, I have 2 questions:
1) What should one expect, a chain falling off once per 500 shifts? Once per 5000 shifts?

2) Should I get a frame made to take a triple? (And no, please don't tell me to get stronger and go to a compact crank! - have a bad back)

thanks in advance