clutch is slipping, then grabbing.
You need either to find out what oil is in there as the PO may have put car oil in which ruins the clutch plates, or change the oil completly.
if that fails then you'll need to get to the clutch, first get a clutch cover gasket, you'll need one unles the pO has coated a previous one with grease before fitting.
you won't need to dump the oil, just leave the bike on its side stand.
I assume you know your way round motorcycles?