My bike used to sputter up top too. I was running out of fuel. Even if you have a pingel it is possible that your gas cap is not letting enough air in so you end up with negative pressure in your tank. I just drilled out my cap and it fixed it. My friend's bike was doing the same thing a few months ago and he could not get it to run so he sold my dad his cylinders, pistons, carbs, pipes etc and took the bike to K&T to get the DM put on it and Kim asked what was wrong with it, Brandon told him it was cutting out up top! Kim pulled the gas cap and couldn't even blow through it it was so clogged! So after new plugs different fuel, jet change after jet change after jet change ........ then he spent $5g to get all this shit done, but it was his gas cap. lol! So check that! just loosen the cap and do a run or try a different cap.