Again, I have to go with Cheryl.
Those wierd hard to pin point cases are usually not an actual "lameness" issue but something deeper. You say she had a workup-What bloodwork? I don't know what region your in, but here in the states undiagnosed period lameness with no injury always check for Lymes disease.
Also possibilities;Head trauma,wobblers syndrome,EPM[equine protozoal myeloencephalitis] EDM degenerative enceph.... EHV-1[Equine Herpes Virus,outbreaks current in US]West Nile-also current in US
Hate to bring such bad ideas but those are a few that aren't as scarce as people think.
I'd ask if they think it could be an obsolete thing, see what their reaction/input is. Its hard to say about work until you have a definate answer for the issues at hand. Hope it all works out.