I am not an expert but my jack russel did exactly the same thing and so did my friends jack russel , i took my dog to the vets and he told me its a condition called luxating patella its a bone condition to the back of the knee joint. The ball slips out of the socket which causes them to hop untill it goes back in, it is a painful condition but only momentarily. You can have it operated on but it depends whether Your dog is lame more often than not. Dogs that have this complaint should not be used to breed with, it is also common in bull terriers although my bull terrier is fine. I suggest you get a second opinion at your vets. Hope this helps you. carol v