How can i fix power loss in my Gilera DNA 125cc?

Ive had my Gilera DNA for two months now and it was running well until a few weeks ago, when the power would just cut out. I would be say doing 60mph nearer the top end of the scooters performance and the power/speed would drop of too about 30mph yet my throttle hand would be twisted up at the 60mph it would do this for maybe 20-30 seconds maybe a couple of times a journey. It also kind of back fire's either before or after the power drops making a kinda popping noise.

I recently changed the air filter, the fuel filter, the oil filter, done an oil change too and i've put fuel cleaning fluid into the petrol tank. The scooter still looses power though but only goes down to about 45 now not any lower and it only happens when i go over 60 mph ish, if i keep to 50 it wont lose power at all during a journey.

I changed the spark plug recently too and i've read other people having the same problem and people suggested it could be a faulty HT lead?

Any suggestions?, thankyou