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It might help to know morw about dog in question. How old is the dog? Is this a recent occurrence or has the dog always done this? Is the dog in good shape and excersized/run regularly?
You might want to have the dog checked for heartworm. It might just be a case of building up it's stamina with more time afield.
Sunny is right I bought one that did the same thing, his prevois owner had him checked and found he had low pottasium. I give atleast 10 mg/day seems to be okay.