One would think it would be carb related if it runs up well otherwise. How low are you trying to go? If you nudge the base idle up a tad, does it seem to hold and be responsive? Some of these can also get a noticeably worn throttle shaft that can contribute to this. Walbro or Nikki? You could test your intake manifold and seal for leakage with bursts of jet spray. All that being said, could it be the start of something valve train related? A partially sheared flywheel key? Honestly, other than the throttle shaft or debris in the carb- this is an odd issue to have.