Sprint St 955i 2004

Hi all,
This is my first post so some of the questions I my ask may have been awnsered quit a few times.
First of all there is a big clunk when I engage first gear and second gear. When warmed up the clunk can happen upto fourth gear.
I have changed the oil to a good fully synthetic and readjusted the cluck. I have also adjusted the chain slack but the clunking remains.
Could this be an indication that the couch needs changing ?

Sometimes when I am in a high gear and the reves drop and I excelerate the engin plutters.
Any advice appreciated

 

Thanks for replying. Ah yes I did have a few last night as just finished 10 days in work.
I don't have any sevice history and the sprockets and chain are in good order, no evidence of wear on sprockets.
When on the centre stand the rear wheel doesn't move with the couch leave fully held in.
The mileage is 34600