Not accelerating past forty mph

My car will turn on perfectly, run, but when I try to accelerate past forty mph, the revolutions drop which reduce my speed, so I cannot get past forty mph. Also, when I am parked and the car is on I can smell gas. Check engine is on.

I replaced the following:
Spark plugs.
Spark plug wire set.
Mass airflow sensor.
Throttle position sensor.

I added the following: transmission fluid.

I was told by AutoZone it could be the throttle control solenoid, idle air control valve, or that it needs a cleansing of the throttle body.

By the way, when I try to use a computer to read what is going on, I cannot get anything but that has not stopped me from driving past seventy mph on the highway before. What else could it be?