I thought I needed a new CDI for the Thruxton. It would start and idle fine, then as I rode the left cylinder would quit, did all the things you did. I would sometimes get a spark on the left plug. Turned out to be a stuck float in the left carb, it was flooding the left cylinder to the extreme. The stuck float was discovered by accident, I had left the petcock on over night, the next mourning I had gasoline running down the driveway from under my garage door. Gasoline was dripping from my exhaust. Yes I felt pretty stupid, hadn't ridden the bike all summer because of the cost of a CDI.
