Ok so on my 72 Roadrunner the speedo was making alot of noice and the needle was bouncing around alot. I changed the cable because the old one hardly turned when it was out of the car. Went to a car show yesterday and it did the same thing. I stopped and unhooked it under the dash and it didn,t do it from there on . Any idea,s what is going on???

Try cleaning and lubing the cable core.

what lube do you think is best to use?

Your parts house will have some specialty lube for it (graphite based iirc). Did it start out of the clear blue (with no changes in that system)? If lubing the new cable does not relieve it then take your old inner core & cut it to a manageable length & use a reverseable drill & a helper to hold it & spin up the speedo (CCW) & see if the speedo head is the culprit.

Graphite or a very light coat of white lube