My unit runs exactly the same as yours does. Runs fine right up to 29-30mph and then goes fffppppptttt 4-stroke crazy.
It will then slow down to about 26 and then clean out, running back up to 29-30.
IAs an experiment I pulled the whole filter out of the bike and whoa!, it ran right up to 32+ with no 4-stroking.
This tells me that the intake filter is overly restrictive, not letting enough air thru to the engine. I am now looking for some of that green filter foam to use to see if it makes any difference over the stock sponge.
I will let you know.