Same problem I had. I spent over $2000 on it before finding the cause. It was a bad multipurpose ground on the transaxle housing. Pull the air cleaner assembly off, dismantle the ground, clean it all up, and reattach it all. Be sure to disconnect the battery first. This ground is a pain to get to, and you have to use a backup wrench on the larger post hex in order to unscrew the pressure nut, or you'll rip off the connection eyelets. Ask me how I know...
The ground post is located under the air cleaner, and under the crossover exhaust pipe.