When my Roadster acted similarly it was the battery getting weak. Next time it does it try putting it in gear, pulling in the clutch, get the bike moving backward and drop the clutch. Don't like coast it backwards down a hill and dump the clutch, you just want to back the engine off compression. That method's got me home before from hundreds of miles away.
Also make sure your main 30 amp fuse is good, clean, and a tight fit in the fuse holder.
Good luck!
Edited to add: on second reading I see you've tried swapping batteries, so maybe I'm talking out my ass. Keep us in the loop, please.
P.S. Have any more pic's of the Tribal?