check your crankshaft and piston arms and bearing seals for any worn parts because i had a peugeot speedfight with the exact problem run for 2-5 min then just cutt out and as i went deeper in to the engine i was the crankshaft bearing that had gone because it depends on how many mile you have on the clock because they are only ment to last for about 6000 - 7000 miles on gilera runners.
adam smith bike and car mechanic
June 2010