I have a 350 verado on my Harrris Crowne. Has around 20 hrs on it. Started running a bit rough. Took in had bad pencil coil, replaced it and spark plug, put back in water and running even rougher. Will run 6500rpm on top end. And around 52mph and smooth. But when back at low rpm and idle she will shake and sputter! It’s not giving me any codes or anything. Just running rough. Help me out please.