Did you actually take it apart and clean each piece, then re-lube with a light oil? My XTR rapid fires started acting up. I took them apart, carefully, and sure enough, the original grease had gotten hard and yucky. I cleaned each piece with WHite Lightening cleaner, and used shimano oil, and they are like new now. Just spraying stuff in there won't do it. I'm assuming your shifters have screws to take them apart. Be patient, there are quite a few pieces in those shifters.