I have a 7 year old Shitzu that has the same problem that developed about a year ago. The first vet that I see told me that it was a neurological problem and they could do a surgery that would only help him for 3 to 6 month. I didn’t like this answer and the other option the vet gave me was to put him down. I wasn’t going to do that to my baby. So I talk to another doctor at the same clinic and he told me that he could have arthritis and to try putting him a dog box to limit his movement and he gave me pain and inflammation pills. This helped him a great deal after about two week of being in the cage about all day he stopped having problems. Everything was fine until winter hit and he started hurting and flopping his back legs around like a fish. So instead of going back to the vet for medicine I went and bought all the top pain relievers that we take and I started give those to him. The only one that didn’t make him sick and helped him with the pain was Advil. I give him a pill every other day during the colder months and in the summer time I only give it to him when he starts hurting. Really limiting his movement has helped the most. I carry him down the steps when I let him out, and I carry him back up the steps when he’s done. I hope this helps.
I don't have any clue, but i also have a dog who has this same problem. He has had xrays and nothing shows abnormal, his pain and limp seem to come and go for no apparent reason. I have noticed a connection to it and if its raining or not? I really dont know but let me know if you find anything out! Shannon