It may not be the same thing and I didn't mention it earlier because of that but, here goes. Stormy, my Cayuga drake, will sometimes do something very similar to what Mustard was doing. I've pinpointed the cause to feed. I believe he chokes on his feed by scarfing it down way too fast, even though he has food available to him at all times throughout the day. I had to change their feed from Flock Raiser Crumbles whenever they were about 6 months old because it was starting to happen too often. I know you feed Purina Duck Pellets, but is there a chance she choked on some of the powdery remnants or a smaller pellet? Stormy usually does this for about an hour, sometimes more, sometimes less. This may be completely different from what was going on with Mustard, but I wanted to offer a little bit of hope.