When driving my 99 Chevy blazer LS 4wd automatic transmission the rpm bounces up and down. I think it is the transmission i feel it disengage and re engage slightly type thing kinda like it’s slipping. Mainly going down like an 45mph road but going fast like 55-60 it rides a lot smoother and stays engaged. I think I heard it may be the automatic transmission torque converter clutch pulse width modulation valve. Any help appreciated.

 

If you know the speed is constant yet the tachometer reading is not also constant, the tachometer stepper motor could be bad.

I had something similar happen to my speedometer in my 99.

I replaced the whole cluster with one I got from a wrecking yard. The only inconvenience was that the odometer was now off.