I wish I could give you a simple answer, but I can't.
If it is four stroking at high speed, you most likely do need a smaller jet. I notice you seem unsure of this, if it starts four stoking, you get an immediate loss of power and a very distinct change in engine note.
I'll describe your other problem as hesitation off idle. Does this happen if you roll on the throttle more slowly? Do you have an idle mixture adjustment and if so, have you tried turning it in or out to see if that makes a difference? Does moving the needle from all the way up to all the way down make a difference? If so, what?
Sorry I can't be more specific but these are the tests I would run if I had the problem.
Btw, have you made sure your timing is right and your carb has no crud in any of it's passageways?