I had exactly the same problem with our 750ss a long time ago and I found that an oil flush and change and new clutch springs (£7.11 the set !) cured the problem. I always use Shell semi-synthetic oil and the problem never came back.
On my DD racebike I went the whole hog and changed the plates too (£60) and that stopped the slipping I was getting.
If you do it yourself, just make sure that you don't lose the green O-ring that goes between the cover and the crankcase. It normally remains stuck to the crankcase so is best left undisturbed.
Alternatively you could flog yours back on fleabay and buy a decent one
