as mentioned, the only thing happening when only shifting to smaller chainring is that the chain becomes slack for a moment before the chain tensioner comes into play. i'd look at that, as mentioned. it might be THE opportunity for something that is misaligned to be making it's move.

and i may be way off base here, but i suspect that many problems that can't be reproduced on the stand are due to the fact that there is less slack for the tensioner to take up on the stand, in that people rarely pedal a bike with an unloaded wheel by hand as they do when propelling it along the ground with their legs. leaving slack on top and the bottom. the slack is more equitable above and below the chainstay. just a thought. could be way wrong about this.