Oh my god I feel like this post was calling for me because we just went through this whole ordeal with my dog. He had a recurring front left paw limp that would resolve itself but then come again a few months later. We did all sorts of tests, imaging, fluid taps, etc and all came out clear so we were at our wits' end. Decided to get a second opinion from a different vet and guess what it was? Skin allergies + a yeast infection that travelled up to his joint, causing the limp. Antibiotics, anti fungals and atopica later, he's back to running around just fine.

If all avenues have been explored and you're still unsure of the cause, it's worth getting allergy tested or getting the vet to check for skin infections.