Mine did too. So I checked the drives, and the cables, and it still twitched. I had it refurbished, it still twitched. It did it after about 70 miles. First it twitched, then went to 160 mph and stayed there. On restarting it zeroed, worked for the next 70 miles, then went back to it's old bad habits. Difficult to ignore a Shadow clock...
David Woods diagnosed a sticking / worn out escapement. This sounded plausible: the escapement engages at a fixed TIME interval and "counts" the revs since the last engagement, and thus calculates speed. If it engages more or less at random when speed is steady, then the speedo twitches: the calculation is accurate, but a key part of the input data isn't. The good news was that he had had new escapements made. I bit the bullet. My speedo doesn't twitch any more, and as far as I can tell, although it's less important than the recurrent attention-seeking semaphore, is accurate within about 5%.
(Acknowledgement to Jacqueline Bickerstaffe and Eddy Saxton: they know a lot about speedos, and I managed to pick up a little. And to David Woods, of course.)
