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Hi, this is Dr. Gabby. The first thing that comes to mind is Syncope. Syncope is where the heart is having trouble getting enough oxygen to the brain which causes fainting. It would be more like to happen when he was exerting himself and it is also common in his breed.
You can read more about it here: http://www.petmd.com/dog/conditions/neurological/c_dg_syncope#
So he needs to have a good veterinary exam including heart x-rays, possible heart ultrasound, and blood work to rule out a metabolic issue. Because there are a number of causes it is important to determine the cause before any kind of treatment plan is made.
Until he sees a vet I would not take him to the dog park or let him exert himself.
He could also be having mil seizures but when they seizure they act "out of it" right before it happens and are "out of it after it happens." There are not back to playing 5 minutes later.
Does that help? Do you have any additional questions?
Ok, so he needs a cardiac work up with an ecg, chest x-rays, and ultrasound. We would not diagnose any particular condition based on an irregular heart beat. Just call it an arythmia for now.