Could be a number of things but we need more info first. 1. Has this always happened since you have owned the bike or has it just started happening? 2.Have you made any changes recently to any part of the bike? 3. What is your maintenance schedual like? How long since the bike has been pulled down?
I could take an absolute stab and guess it is a carb issue possibly a blocked jet, or a power valve issue or an electrical issue but we realy need some more info.