Laura:
I am flattered that you think I can diagnose what is obviously a very strange and difficult, though not necessarily serious, condition. Certainly porphyria is one possible diagnosis to consdier in a case like yours. If the third biopsy is consistent with that diagnosis, then perhaps repeating the urine test might be useful.
One thing dermatologists often do if they are affiliated with major teaching centers is to share cases with academicians and colleagues at a "Grand Rounds" or "Staff Conference." With the history and laboratory tests posted on the wall for reference, other dermatologists examine people like you for an hour or so, then sit down later to discuss their collective expereince and opinion. If you were my patient and I were stumped, I would arrange to have you seen in this way, which gives you in effect dozens of (free) second opinions.
Perhaps after the next set of tests come back, you may want to discuss this or other avenues of obtaining additional opinions.
Good luck.
Dr. R