Check the crank positioning sensor
what makes it worse is there is no testing procedure given for the cdi box. Only replacement instructions. Something like unplug, remove, replace, replug. So obviously if everything is testing good into the cdi box you are going to assume that is the problem. i wonder how many good cdi boxes have been replaced over the years.I worked on a 400EX once that the only part that tested good was the one that was bad.