Triumph Repair: gt6 spark lose problem, ballast resistor, condensor

Question
i have recently put a gt6 engine in my spitfire, that was fine has been running fine for 3 months, now it will not start or if it does start will not run for long. replaced cap, leads,plugs, condenser and points but the engine seems to lose spark after the engine runs and vibrates. the shaft in the dizzy moves up and down. seems like the cam or dizzy moves and it loses timing or consistent spark

Answer
If it ran OK in the beginning, then I don't think its the distr or cam.  Likely the replacement condensor is bad, the coil is going, or needs a ballast resistor, or the wiring for the points has an intermittent short.

I would look for a wiring problem first, something grounding out when it shouldn't be.  The intermittent nature of your problem points to that.