99 Gt Misfire ??

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99 GT automatic runs great idles smooth but around 2500 it starts to shudder. Took it to the trans shop thought it was in the trans . He ran test said it was a high speed misfire not the trans. If I put it in park and slowly rev it up it starts to shudder a little. I put on new coils all 8 and new plugs no difference. What else could it be, next I will take it to dealer and let them do a check but really dont want to....