I think the OP corrected himself, it's about 2mm between the cage and the chainring. Still, for me, about 1 mm too high, but I don't think that's the problem.

I'm guessing here but some other things you can check.. remember, all gueses:

1. Chain length - I run mine long and unscrew (if applicable) the b-screw all the way. This causes the chain to bounce a bit, but I deal with it. The chain bounce could be causing some of that jamming. A worn chain bounces a lot more than a new one. It also derails easier.

2. Chain line. If the cranks are too far out then it could cause problems on the inside.

3. Angle of the front derailleur. My outer cage is parallel to the chainring.

4. Cable hanging anywhere? Meaning under BB or in the bar tape area?

fwiw I double shift all the time, no guard, almost always no problem (dropped my chain maybe 2-3x in 200+ hours of riding).

I prefer the N-Gear Jumpstop with its metal cage vs the hose-clamp-and-plastic thing formerly sold by other folks. The plastic will break eventually. Hm, I should dig up my Jumpstops and see if the large one fits on my Cannondale's round seat tube.

cdr