Listen to MinnFlyer & gaRCfield.  They are right on.  I have had this exact same issue w/a Saito 125.  Leaning the low end (a bunch) solved the problem.  Funny thing is that it took about 6 gallons before this "issue" appeared.  And, I had already leaned the LE a considerable amount during/after break-in. 

But now it'll quickly settle into a nice solid 1700rpm idle.  And transition hasn't changed...which is good because it was instant even before leaning.