If the pup has not been tested for Lyme's disease, she should be. This tick borne illness can cause symptoms as you describe, and is getting more common across the us.
At 8 months, 75 pound is heavy for a pup. They are skeletally immature for their weight. If she is going to be a very large dog by breeding, and is not fat right now, I still suggest you switch to an adult formula food. She should be restricted (under direction of a veterinarian) from growing so fast. Her growth plates need time to close before the full weight of muscle is put on. She should not only be kept from excessive weight, but this kind of dog who is already showing orthopedic issues should be kept underweight.
It doesn't sound like she is over-doing the exercise just before the lameness appears, but if she is, make sure you give police her play. She should never reach the point of fatigue, as her muscles provide protection for her joints only when they are strong.
If this keeps happening, I would get a referral to a specialist. Or at least have the x-rays sent to a radiologist for interpretation. These things can be very subtle and easy to miss on x-ray.
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Dog Veterinarian
20 years experience in veterinary field.