I to have one that has suffered with this since a pup ,mine an English pointer , I have tried everything from sugar water ,power aid mixed with water afield ,to snacks he wont eat anything while or during a hunt,seemed if he is on the ground for three hours it might happen at any time I also carry nutrical afield ,he is fed only once per day at night a high quality feed ,lots of protein and good fat content he is a lean fit English ,now I have had just about every test ran know and the vet tells me its simply a sugar drop ,seems to happen when its warmer out ,or if he is stressed while on the road , not eating as much ect. this year I'm trying something different , I have been giving him the squeezable baby food pouches as he comes in for a drink its the first thing he will take while out hunting time will tell but i'm hopeful he really likes the banana and yogurt , the little pack has 17 grams of carbs ,ect . I have his litter mate a sister and not one time ever has she show this trate, she runs just as hard in a day although she usually a couple miles short on what he has done in a few hours. its Scary as hell when it happens ,I can usually tell but its to late when I see it ,it comes on that fast . thus far usually in 10-15 min he is ready to go again like nothing happened but he is leashed and we are done no bird is worth loosing a dog over. what breeds are you experiencing this with just wondering??