I am trying to put the 600 on the market, but I have to get it running right first. After running perfectly, the engine stumbles, then comes back just as fast. Sometimes, it stalls completely. This is while idling and not moving. I remember seeing somewhere that the ignition system is at fault, and that it is common on older XRs. Is it the CDI? Coil? I don't want to replace everything trying to trace this out.