Terrain's speedometer is bouncing and I don't know why

2012 Terrain V6 Automatic 188000 miles
My speedometer bounces...flutters...? I don't know how to describe it. I've been to 3 different mechanics and had an exhaust back pressure test. They were also stumped and wanted to do more tests. Does anyone have advice? I don't feel like I'm being drug along but these tests cost money that I don't have.
It only happens every 10mph.

Here is a video