Eric27 They have the rubber cups make sure they are tight. Also cable free with no kinks. But they all vibrate a lot you just learn to live with it. You usually find when the bike is riding it smooths out a little. Regards Joe.
As stated above check the run of the cable is smooth ie no tight bends. failing that try putting an old bit of inner cable on a battery drill and run it. that will tell you if the speedo itself is knackered.
From memory they changed from Smith's Speedos to Veglia at some point( I think). There's a reason we all used to refer to the latter as 'vaguelier'.
