Popping it seems to me is always a man made thing brought about by excessive use of the stop whistle or other forms of correction causing a lack of confidence on the dog's part. He expresses that lack of confidence by stopping and then looking back to the handler for reassurance; hence "popping".
So, the simple answer is drop the corrections at least pro tem, and build up the dog's confidence to the point where popping disappears.
One way would be to run a graduated series of cued "memory retrieves", then transition into very easy, but nonetheless genuine, cold blinds.
I'm sure you know this, but for them as don't ...
With the dog at heel, drop a dummy into a very easily identified spot; might be at the foot of a tree, by a fence post, bit of tussocky grass, traffic cone, whatever. Walk on a few yards and send him. Over time build up the distance. When he's running with confidence do the same dodge, but from a different angle. Then from all sorts of angles.
When that's down, unseen by the dog, salt the spot and run him from a couple of feet. Continue but progressively add distance and angular variation. Then in a bit of fresh cover send him on a genuine un-cued cold blind from a few feet. Make sure he'll find something by planting multiple retrieves, up wind; if you've got a few fresh shot birds so much the better. At this point you'll hopefully have run a full monty, albeit simple, cold blind without a pop. Just build up from there.
Best I can do from this remove,
Eug