My 22 month old boy has the same problem ever since he I got him at 10 weeks of age. I just spoke to my vet about this earlier this week. She says its normal in the case of my boy. She does not recommend strenuous exercise when temps get above 80 plus degrees. She told me to regulate his activities and have plenty of water to cool him off. It is strange that my other two Dobes did not have this problem.