Thanks for the replies. FBinNY, you caused me to take a closer look at what's actually happening -- at least on the stand. To my surprise, the mis-tracking ISN'T happening when I shift the rear derailleur. It happens, under certain conditions, when I shift the front.

With the chain on the 4th or 5th sprocket (counting from the 11T) and the big ring, if I shift to the little ring, the chain tracks off the idler to the inside (small chainring side). It's off track at initial contact with the idler, but is on the idler within 1/4 of the way around. If I back-pedal even an inch at the crankarm, the chain gets back on track.

The jump from 50 to 34 up front is apparently allowing enough chain slack to let the chain pop off the idler teeth. In that nano-second of slack, the fd is pulling the chain off the idler to the inside. At least that's my theory. Anyone know how to fix it?