Carlos... I suspect several reasons for the jumpy needle.
1. The air temperature... it was cold 40's (F)... it hardly does it at all when it's warm.
2. Cheap aftermarket cable doesn't like the cold and it could use more graphite added... or just replace it.
3. The bend of the cable may be too sharp in some area(s)... I'll have to review my routing since this has a recently transplanted Muncie M20.
4. Rapid speed changes as seen in the videos show how it jumps around wildly... especially at the higher speeds.
5. 60 year old speedometer.... wear and technology are not it's friend.
When I was on the highway cruising at 55, it was not jumpy at all... only when thrashing the car around was it jumpy.
I use the speedometer mostly to give me a "General Idea" of my speed... mostly.
For me it's part of the nostalgia too ... I like it... yeah... that's it.
If I'm concerned about my actual speed... I pull out my phone and use the Thunder Speedometer GPS app.... works great.
With the info from that app, I was able to find out that I need to get a set of those colored speedo gears for my M20.
At exactly 59 on my speedometer, I am really doing 55.... need to pick the right color/ratio gear.
When I get those gears I'll probably just put in a new cable too... which I suspect is the most likely culprit.