surface

Hi, I'm new on here, and have little bike knowledge, wondered if one of you more experienced guys could help me out. I have a 02 Sherco 250, it runs great but I have one problem with it. It starts first kick but when cold if I select a gear it jumps forward and stalls. To get round this i have to perform a rolling start, before selecting the gear. Once its warmed up I have no further problems. Have you guys any idea what causes this and if so what do i need to replace or fix? Any advice would be much appreciated. Cneers