SPEEDFIGHT 2 LOSS OF POWER PROBLEM?

Hi there, I have recently bought a dodgy speedflight 2 2006 reg, I got the bike realy cheap as the owner didn't know what the problem was with the bike. Whenever I start the engine it ticks over fine but as soon as I pull the revs back the revs shoot up and then after a few seconds the bike dies right back down to virtually no revs again and just about ticks over again If you pull the revs back again it just bogs out and eventually cuts out. If I start the bike again I have the same problem. I have cleaned the card and put a new air filter in. Would it be something to do with the fuel mixture being wrong Or could it be something more serious like the piston ring etc..