It certainly sound like a clutch problem, & this is definitely the place to start.
Since the bike has stood for a couple of years, it may be that the clutch is rather gunged up with old oil & muck. The first thing I would do is dismantle the clutch, clean all the parts well & examine them. If the clutch is gunged up this could explain a lot of the problem, but you must check the centre & the drum for notching. All the grooves the clutch plates slide in should have smooth sides, if they are notched (serated looking) then you must either file them smooth or if very bad think about replacements.
Hope this is helpful, & good luck.