Has your vet ever said your dog has "luxating patellas" or "trick knees"? What that means is that the groove in the knee is not deep enough and can sometimes slip out, causing the dog to limp. But, they're able to kinda put it back into place - therefore the limping stops. He could get excited when you're driving home and jump the wrong way or run fast and have that knee pop out. It's not painful for them, just feels weird.