it could be a couple of thing causing it,
is your clutch cable moving freely ?
if you loosen the adjuster near the clutch arm, you should be able to unhook the end of the cable.
take it off and check that the cable moves freely up and down inside the cable outer.
if not it might need a clean out and some lube, i use Penetrene, it gets in well.
if you unhook the cable at the lever, then you can pour some in to the end and let it run down.
after a minute or so you can add a little more.
then stop once it starts dripping out the bottom end of the cable.
the other thing it could be is a worn clutch basket
they wear on the alloy tabs that locate the clutch plates, grinding grooves into the basket.
then it becomes sticky/notchy every time you pull the clutch lever in.
the only real fix if it is this, is to replace the clutch, you may get away with just replacing the basket though ?
http://www.ebay.com.au/sch/i.html?_...at=0&_from=R40&_nkw=ds80+clutch+basket&_arr=1
the other thing it could be is that the clutch springs have lost tension,
but then my clutch would start slipping at higher rpms too.
mine springs were pretty weak, hardly any effort was needed to pull the clutch lever in.
my spare DS80 engine had a set of heavy duty clutch springs in it, i swapped them into my clutch and haven't had a problem since
but the clutch lever is now pretty firm to use, you need to get used to it.
http://www.ebay.com.au/itm/83-00-SU..._Parts_Accessories&hash=item19d96ac5bb&_uhb=1
if you need to remove the clutch be carefull,
gooseman snapped a leg off the clutch hub when he was undoing it iirc ?
this is the proper tool for holding the clutch to undo the nut.
http://www.ebay.com.au/itm/83-00-SU..._Parts_Accessories&hash=item5d3b321c06&_uhb=1