OK, so if I'm reading & understanding correctly: Happens at a light decelleration, butterflies partially/mostly closed. At that point, should have a pretty high vacuum reading, not pulling much fuel, no load. Maybe it's a bit too lean @ cruise, or maybe advance weights are sticking (not sure what ignition you have). When you say "missing", does it feel like chugging, or dropping the occasional spark, or like a burble from the exhaust ?
