Does it "bounce" between 50 & 70?? If it does the speedo cable needs "lubing" up, it's gone dry & will cause the needle to waver!!! Unhook cable at the transmission, pull out the "inner" cable, & wipe some white grease or "never seize" on it, then slide it back in. You will have to "twist" the inner at the very end of shoving it back in outer sleeve to get it to connect into speedo head!!! & re hook it to transmission. Should fix the problem.

Didn't GM used to sell a lube just for speedos? They likely dropped it since they are all electronic now...