Interesting… I have a similar issue on the 5.9 in my Barracuda. I am pretty sure it is due to collapsing lifters when revving past 4500 then decelerating. In my case it is my fault because I did the pushrod measurements wrong before ordering new Smith Bros. Pushrods…urgh…. So I used the stock ones which are a bit to long .
Your issue might just be dirty clogged lifters ? Take them apart and clean them .