front leg gives way

Sometimes when my dog is walking on uneven ground, her left front leg looks like it gives way. She holds it up and limps for a few steps so it must hurt, but then she is back to putting weight on it and seems fine. This can happen as often as once every few minutes. I can't see any swelling and there are no cuts.
Does that immediately call to mind a possible diagnoses? Shall I wait to see if it improves on its own or should seeing a vet be more immediate. Despite her age, she moves well and does not seem or act old in any other way.

[Justine Lee, D.V.M., DACVECC]

I'd have her seen at your vet. If she doesn't respond to a pain medication (non-steroidal anti-inflammatory dog medication) after a few days, she should definitely have x-rays done to rule out a bone cancer called osteosarcoma. This is usually very painful, but can occur in older dogs.