After much in-depth internet questions and answers, it seems that Payton has congestive heart failure. This occurs in dogs who have been heartworm positive. Payton was heartworm positive when I rescued him at 3-1/2 years old. He had the treatment and has been on heartworm preventative ever since.

He is doing OK and does not seem to be in any pain. His episodes have decreased in strength. He just deals with it for a few minutes, and then it is over. They used to last 30 - 40 minutes. We're hoping for the best.