Thanks for the update, wellandfit. When you first described your problem, I went immediately to the rivets as the problem, but if it were my bike, I wouldn't start that repair before doing all of the other easy things first. I just wanted to know if you'd done all the easy things before concluding it's damper plate/rivets-related. Now that we know those efforts didn't correct your problem, like others here have said, you need some clutch parts, starting with the damper plate. Number 6 in the picture.
Do you need new friction plates (numbers 3 & 4)? Maybe not, but as long as you're in there, replace those, too.
There may be other things you find that you need after getting in there, but I wouldn't buy them blindly.
Be sure to get a new gasket (number 2 in the picture). You won't be able to get the old one off in one, reusable piece.
Good luck and thanks for not burying the bike!
