I have the same problem.
In an effort to decide what causes this, I took the speedowire off the front wheel and connected it to my Dremel instead.
Ran the speedo up to 160 kmph (100 mph) and the needle was rock solid.
Did the same thing with the tacho and got the same result.
Conclusion:
It´s not the wires that causes the flutter in my case.
The wire drives a magneto in the instruments.
When the magneto spins, the magnetic force causes the needle to swing.
The needle is held back with a tiny spring, and at speed it´s the equivalent force of the magneto and the spring that shall keep the needle still.
This works just fine when the instrument is still, but the vibrations of the engine, bumps in the road and other irregulaties upsets this equilibrium.
On an instrument that has spent so much time on the road (as Hondas do...
), the clearences are not perfect anymore and that only makes the problem worse....
My 2 cents....
Cheers
Daniel