how can i fix 2003 gilera dna 125?

3yrs 3mecanics and cant wirk out a problem, when i start it she ticks sound but when i pull on my thottle it farts and cuts out. if i leave it to tick it will tick for ages chucking now and agian, but once i set off on it every time i turn the throttle it just feels like its not got no power it feels dead but then it will shoot up and then no power agian. when i stop at lights i sit there it ticks over but when i go to go it just cuts out and then i cant start it for ages. then eventuly its cuts out for good till i take it to mec but he gets it started and says ive fixed it but then 2days later its the same. i have had a new ht coil put ,new spark plug and lead. air filter cleaned. please its doing my head i badly