It could be a long shot but we had video issues because of a PCIe lane issue as pointed out by @didikunz in this topic.
The Decklink turned out to be using less than the 4 PCIe lanes and caused stuttering issues over a longer period or when adding more than 1 channel. Might be worth checking if you Decklink uses all of it’s PCIe lanes. Ours turned out it did not which caused all the issues, moving the Decklink to a different PCIe slot fixed it.