STX 12F won't stay running.
 

My 12f was running perfect last weekend and then suddenly lost power and died. It sounded as though a plug went bad and lost a cylinder. So I replaced all four plugs, started it up with the seat off and it ran better than ever. Put the seat back on and it did the same thing again and died. So I removed the air box thinking it was somehow clogged and not getting enough air. The box was clean and so was the throttle body. So when the seat is off it runs perfect.....put the seat on and it dies. Any thoughts??