Is it electronic ignition? if so; I would check and maybe replace the ignition battery it sounds like WIBS, weak ignition battery syndrome. I would check the hal sensor wire and screws, make sure the screws are tight and there are no breaks in the wiring. If all is well, dissemble the carb and clean it well. Start off with both screws out 1 1/2 turns and tune it from there. JMO.
Good luck.