Like brightmount my experiences with intermittent lameness involve a TB. He constantly bruises his soles to the point where he will barely weight bare on the affected foot. We now have silicone pads put under the shoes every shoeing. This keeps him sound - without them he is never sound for more than a few weeks at a time. TBs are known for having very thin and sensitive soles. However, if your vet and farrier don't think it is bruising and x-rays are clear I'd push for an MRI. My TBs intermittent lameness turned out to be a combination of the bruising (as mentioned above) and chronic wear to the collateral ligament. This can only be diagnosed through MRI. I'm not saying this is by any means what is wrong with your horse but simply trying to illustrate that you need to push your vets for a diagnosis. Good luck.