This is almost always an indication of a bad file. The corruption could be in the original or it might be being introduced anywhere along the rendering path; in the read, the the processing or the write. If the specks are in the exact same place each time you view the effect, it’s happening upstream of the write. If they move, it’s in the read.
Impossible to diagnose from here, it’s a binary search, though. Try setting your render scratch to a different drive. Try replacing the original file with a fresh copy. Try moving the original file to another drive.
bogiesan
This is my standard sigfile so do not take it personally: “For crying out loud, read the freakin’ manual.”