I wonder if it could it be reverse sneezing that in certain circumstances is severe enough to cut off his air supply that it causes him to pass out?
I was just dog sitting a dog that has reverse sneezing but usually 1 minor incident per day/two and it kinda sounds like a hacking/cough
Google videos for reverse sneezing in dogs and see if it sounds similar. If you could video tape a minor incident that Nicky has it would be easier to describe it to your vet.
