That's a tricky situation similar to mine. Asked with a two-stroker friend, and he said it is because of poor tuning or lean mixture plus choke acting up. Mix will be lean and a sticky choke will make it rich behaving normal and then when the choke closes when it changes its mind, the mixture will be too lean causing the power drop or even engine shut down (which is happening to me). He asked me to block the choke inlet and tune. Yet to try that.