1995 Camaro 5.7 liter Z28

I bought a 1995 Camaro Z28 5.7 liter for my son. It has 205000 miles on it. I have changed the spark plugs, wires, alternator, battery, fuel cap, fuel filter, air filter, mass air flow sensor, catalytic converter, and 02 sensor. While I am idling with the air or lights on or off the battery gauge shows about 14 volts, as soon as I give it gas the gauge shows about 10 volts, right at the top of the red on the gauge. I can go 60 mph on the freeway and the gauge slowly goes from about 10 down to where the check engine light comes on. If I coast at this speed or any speed the gauge shows about 14 volts. Any direction you could send me in would be helpful. By the way the check engine light will go off if the gauge goes back up to around 14 volts. I have tried to have it checked at AutoZone but they say they cannot get a reading of any kind off of the plug in for their diagnostic reader.