I had a similar thing happen to my 300 while pressing it hard on a sandy mx course. I identified it as a lean out condition, caused by too little flow through the hose mounted ball valve on the fuel tank cap. You know the one that is supposed to stop reverse fuel flow if you drop the bike. This got worse with a topped up tank, as the small air pocket made the pressure drop too fast and preventing proper flow to the carb. When i removed the valve the symptoms were gone. I had a long track side mech session finding this out, tearing down every part of the fuel flow and throttle cable. This never happened in my normal woods riding of course.
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