i had the same problem in my rst was unable to go over 5000 rpm without missfiring like crazy and if i squized the accelarator slowly it was good, try checking youre crank sensor, it should be counting ignition from the flywheell, a littel carburator spray on it did the trick for me, in case you'rs got dirty it should help. if not the try replacing it ....

good luck mate !