Hey guys, I need your help with my bike as it is just marginally ridable now and I'm out of ideas on how to fix this problem.
The bike is a Trek 5200 (Ultegra 5600 triple up front), with about 2200 miles on it. What I'm seeing now is that anytime I put power down, i.e. starting from a stop or going from coasting to full sprint, my chain snaps off of the middle cog in front. It either ends up on either the small cog or completely off on the inside. This only happens on the middle cog up front, bigger and smaller ones I can't reproduce it. I do about 50% of my riding on the middle cog, 40% on the large cog and about 10% on the small cog.
The only thing I can think of right now is that I changed the chain recently without changing the cogs, but I'm skeptical because I was able to do 3-4 long rides without this issue. Also when it started, it was fine in the morning when I commuted to work, then it started doing it on the way home about 2 lights from home (its done it on every start since then).
I also confirmed its not a front DR issue. I tightened the cable and it still does it and I also tried overshifting (so the chain *almost* wants to jump to the large cog) and it still is easily reproducible.