Speeo.Quest?

72 dart 318/904. Replaced speedo. cable a few years ago(less than 1000,miles). Odometer(miles driven) works good.Speedometer needle bounces around while moving. Driving from shop to house(50ft) it reads 50-60 mph. Car is going less than 10 mph. Does it sound like the head is bad? or thoughs? Cable is tight to head and trans.