These little guys can have back issues that do not show on x-ray or with palpation but will cause pain and unsteady gait.
This site talks about one such spine issue
http://www.petsurgery.com/fibrocartilaginousembolization.html
Another issue might be a bad knee as those can go in and out and sometimes be useable and others not particularly after rest.
Bad knee - luxating patella - are pretty common in small dogs. You might try Glycoflex or Cosequin which are glucosamine supplements for dogs or even the liquified glucosamine shots Adequan if you think that might be it.
This site talks about patella problems
http://www.cpvh.com/Articles/78.html
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