This is gonna sound nuts, but...
My speedo flutters at 60+ as well. Then it was 50+. Yesterday I was going for a ride so I lubed the chain, checked the oil, put some air in the tires, etc. Much to my surprise, the speedo was steady well betond 60mph. Came home and checked that front tire and noticed some odd wear patterns. I pulled the wheel and took it to the shop and had it balanced. Viola! Speedo is rock steady beyond 70mph. Looks like the tire had been "bouncing" and confused the speedo drive.