I can remember having intermintent ignition problems with a Yamaha XT600. The bike would sometimes pop and spudder when throttle was applied. It would stall and not start back up. After a while it would fire back up. The plugs were wet (no spark). The tech at the local Yamaha dealer recomended spraying electronic coolant spray (picked it up at the local Radio Shack) on the CDI. If it started up, then this would determine that CDI electronics were bad. I was told this was a problem with other mid 80's XT600's. It might work for your yz426 to help determine if you need to replace electronic ignition.