I wanted to respond to your question tonight as I am very concerned about your dog. It appears that he is a young dog, and I will assume you have not seen this problem before, just in the last day. If he is going stiff, and not breathing, I imagine he is collapsing or laying down at the time?
I have a couple of thoughts of what could be going on, one would be seizures, and the other would be what we call syncope (fainting or collapse) which can be caused by a number of conditions.
Seizures could cause a dog to "pass out" and appear to be holding their breath. Syncope could look similar, and in some cases, such as with severe heart disease, the pet could truly not be breathing, which is obviously potentially life-threatening!
Here is an article about syncope:
You will read there, that even if the episode seems to pass and the dog then seems fine, you need to seek veterinary care as some of the causes of this can be very serious and potentially life-threatening, such as very low blood sugar or heart problems.
If you have seen this 4 times in the last day, I strongly advise you to have your dog checked at an emergency clinic tonight to be on the safe side. If that is just not possible, please get him in to the vet right away in the morning.
Thank you, XXXXX XXXXX wish you and your dog the best.