Impala Speedo fluctuating between 0-40mph car drives & shifts fin my rpms aren’t fluctuating was told could be my voltage actuator behind the speedometer & should replace cluster

 

Welcome from Florida. Your problem could be the stepper motor for the speedometer. GM had a problem with them and by 08 the problem was corrected but you could have gotten a bad one. They can easily be replaced and it is much cheaper than a new cluster especially if you do it yourself. If you do it, then replace all of the stepper motors at the same time.