I had a very similar experience. We have lost over 50% of our 30 chicks from Meyer (3 different shipments). Some were obvious right away that they wouldn't make it but most were surprises. They were running around eating/drinking fine, poops looked fine, and then dead a matter of hours later. We tried vitamins, electrolytes, ACV, yogurt, egg yolk, and even Corid. Nothing has made a difference.
The brooder is 90 in one corner and 78 in the other so they can regulate their heat. The window in that room is cracked to allow fresh air but there is not a draft since their sides are covered with an open top. They don't act cold or hot. They are on pine shavings that get changed often and are not allowed to get damp.
We got a batch on wed last week and one was weak and had wry-neck. We brought her back from not being about to sit to being able to stand and eat on her own by giving her egg yolk, wet food, and Vitamin E. Then as soon as we think she is going to make it she loses interest in food and won't eat anything. I did notice her crop seemed to have air in it and seemed kind of transparent. She died a little later.