2000 Chevy Impala 3.8L- While driving at a constant speed the transmission will shutter or flutter between RPM's. If I back off the gas, the shuttering will stop likewise if I step it down, shuttering goes away. IS this a torque converter problem? Any ideas- speed control sensor??

FORGOT TO SCAN THE PCM I SEE.
check all fuses.?
not told
fails hot and cold engine or both
car must be in transmission DRIVE selected? right?
and foot off gas pedal (accelerator)_ rpm goes to 2000 or up to 6000 rpm . and or
so driving foot off the pedal and RPM goes too high
or in drive, I step on the throttle (gas) and RPM shoots way high
and Never leaves 1st gear. that is transmission failures.
in DRIVE.
if rpm also goes that high right foot doing nothing and in park
that is engine idle control's failing.
we can not see you drive or how. so answers can be a tad hard.
in what gear? PRNDL (choices)
right foot doing what?
and what SPEED is too slow 20 or 85mph? or?
does tranny fail to upshift?

No, not normal. Without more information, I am thinking about clutch slippage.

now is the car 2017 plated or 2011 plated
as there is a vast difference between new and second hand new to you
Next question is it automatic transmission or CV transmission
With automatic as you increase rpm the car will accelerate and you will feel ratio changes up to the point where road speed = engine rpm for that throttle setting
With a CV transmission ( constant velocity transmission ) there is a torque converter but that is where the automatic action ends
In the transmission there are 2 pulleys with a big belt that can vary the ratios by pulling the flanges together on one and expanding on the other
This gives an infinite number of ratios and is designed to keep the best ratio selection for the torque band of the engine
SO how does it affect your driving
If you floor the pedal on take off, the engine will red line and stay there until road speed builds up
OR you ease back on the pedal and the pulleys start to change the ratios to suit the power band
How to drive it
when you want to take of push the pedal down to around 2500 rpm the car will take of normally
as the road speed increases , ease off the pedal and it will keep altering ratios until the correct road speed is reached for the rpm on the dial
The only catch that you will have to get used to is when going to pass at speed , you tend to floor the pedal to get past quickly but it will not down change ratios until you ease of the pedal and then the car will speed up to the rpm that you are holding it at
IT is different in driving and you will have to learn to drive to rpm not road speed and then it will behave normally ( set the rpm by the pedal and the transmission will catch up and to make it catch up , ease of the pedal in stages , just like a conventional automatic

check for problem drive line "U" joints or cv joints

catalytic converter, egr valve, oxygen sensors. If you have that many miles on it, try loosening the exhaust before the cat converter and try driving it. If your power has come back, then it has failed internally.

Could be a transaxle Clutch Lock-Up problem,
not OD

TCC Shutter -- Torque Converter Clutch [-- Shutter

If that can be resolved with a new selonoid
is questionable
Has to be diagnosed

Your light on the gas & in OD ,but the lock-up isn't
releasing when you slow down or put gas to it ,to
speed up

Mine was doing that also.. We changed the two EGR valves, changed spark plugs and wires, and finally cleaned the Mass Air flow Sensor with carburetor cleaner. That seemed to helped the most. You have to take the sensor off to do it.. you can find a youtube video that explains it. Now all I have to do is figure out why my car's RPM's surge when slowing down.

The transmission might not be shifting into over drive ...

TRANSMISSION SLIPPING NOT ENOUGH FLUID IN TRANSMISSION OR NEED A TRANSMISSION FLUID FILTER CHANGE.IF YOU HAVE PROBLEM WITH OVER DRIVE.CHECK ENGINE LIGHT WILL FLASH ON WARN YOU PROBLEMS WITH TORQUE CONVERTER OR 3 - 4 SHIFT VALVES SOLENOIDS.

Idle air control motor is stuck open or is bad. It's on the side of the throttle body.