The tacho on my RGS started waving about one day, some years back now.
When I looked, I found a small lead weight rattling around the back of the tacho, I could see where it had broken from. Looked like a counter weight to dampen the gauge.
I ended up replacing the innards with an after market one from eBay that had a 3 cylinder setting.
There are some photos here, towards the bottom of the page.
http://www.laverdaforum.com/forum/index.php/topic,41547.15.html
Hope this helps.