Is the intake or exhaust modified in any way? If so, did you rejet to suit?

To me it sounds like a carburettion (sp?) issue, so if you have modified any of that stuff, you'll have to tinker with the jetting.

If it's stock standard, I would guess that your carbies need a clean, and double check the routing of fuel lines, and make sure you have mounted the carbies and airbox correctly and not impeding airflow.