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Read your plugs..

Get on a side road where you can shut it down and pull off the road at a moment's notice. Try and duplicate the problem and as soon as it starts shift to nuetral and shut the engine dowm immediately. Pull the plugs and check their condition. Wet? carboned up? Sooty? It could be a fuel problem and/or spark. That will give you a base line on where to go from there..

Dave