i thought it sounded like a stickyfloat bowl but, i have a feeling you have a bad wire somewhere.
check all of your wire harnesses, especially the ones around the muffler and motor. anything that looks burnt or corrided should be looked at in depth.
sometimes theres a harness that runs right down under the motor, and if one of these wires has rubed threw, the sled will run fine until you hit the right bump and it grounds out.
i had the same problem on my yamaha, and couldnt find it for a month , some rides would be fine others the motor would just die, i finallty found it one night when it died on the mtn, it just happened to be dark enough that i could see a samll spark underneath the motor. and once i lifted the wire up off the piece of metal it had rubed threw on it fired right up.
make sure you investigate these wires like a csi agent, because if you think there all good your next best bet is a 600 dollar stator