defective spark plug

dear all riders of SYM scooters ,
just to share with you all ,
I purchased an original spark plug NGK original set ,
upon install it on my 250 I evo , the engine would loose strokes at low rpm , like misfiring,
then left it on for two week , but upon it I replaced with the original spark plug
with already 25.000 km on it , the scooter begin to work properly , no more loss of strokes or misfiring ,
So just do not simply put on any spark plug , it may do not improve your performances.
Even my intention was not to improve but just change the spark plug as it was considered worn out , instead is still going well up to now.
RGDS
Spartacus