Had a 5mth old shorthair that died in this manner so the trainer says while me and the family were out of town, said she was running heard a squeal and was stone dead in the snow nothing around her no explanation. Still to this day don’t know what happenedI hated reading your post due to the horrible memory it brought back. My weimaraner was only five when one day we left for our run. He made two hundred yards and stopped. For those that know the breed knows that him stopping after only two hundred yards means something is wrong. I gave him a minute and nothing seemed wrong other than he didn't want to run. So i turned around and headed back home, fifty yards later he went stiff reared up and went over backwards. He was dead in seconds.
No real answer to why, but a month prior the vet said he had a slight heart murmur but not to worry. The only other thing about that day is he had just ate before we headed out the door. My friend which has been a vet for over thirty years suspected his stomach flipped.
Said all that to ask is your pup having these episodes right after his meal?