2008 Chevy Impala LT 3.5L engine, RPMs randomly cut, car won't go 40+

First check the fuel pressure with a gauge. There is a test fitting on the fuel rail and you can get the fuel pressure specifications on the internet. If the pressure is good check for an bad spark plug, wire or coil. Pull a spark plug wire one at a time and see if it makes it worse or stays the same. If it stays the same pull the spark plug out and see if it looks ok if it does swap the plug with the next cylinder and pull a wire and see if the miss is on the original cylinder or if it switched to the other cylinder. If it's in the original cylinder change the plug wire with a different cylinder and pull each wire one at a time. If the miss stays in the original cylinder it is most likely an ignition coil. If it has multiple ignition coils switch it with another coil and see if the miss moves to another cylinder. If it moves you need a new coil. If not check the cylinder for compression, if the compression is good it has a fuel injector. If you are not mechanical inclined I would take it to a shop for a diagnostic. It could also have a problem with a computer sensor and may have a check engine light on and it could let you know which cylinder is misfiring.