speedometer

I recently bought a 2004 sport trac with the v6 with 277,000 miles. I know these trucks can last a very long time but I’m having some issues with mine that I’d love some advice on how to solve. One thing is that the speedometer doesn’t work, I crawled under to check the transmission speed sensor...

Since I bought my 2003 Sport Trac XLT back in March 2023, the speedometer has bounced around when idling (parked or in gear), but when I'm driving more than about 15 mph, the needle snaps into place where it should be. When I slow down under 15mph, the speedometer starts bouncing around again...