trying to find this problem. also, because this is intermittent - it doesnt seem like an exhaust obstruction ( certainly possible though, if cat parts were rattling around).
I guess my next step will be pull coils one at a time on left bank. if one makes no difference - than that "might" be the bad injector. if all 3 make a difference - then i have some reason those 3 cylinders are lean. just thinking out loud.