How can i fix my Piaggio Nrg EXTREME 2000?

When i start my Piaggio Nrg it stutters and i have to sit there revving it until it stops stuttering, then when i'm going along it stutters at certain speeds.

I took it to a shop at they didn't know what was wrong with it and cleaned the filter out and it worked fine for a couple of weeks but it just started stuttering again?

What can i do to fix it? and how can i clean it all out myself and save myself £60?

Harry
August 2008