91 deville going crazy

Hello everyone, I need some help. My 91 caddy with a 4.9 is acting crazy. It has about 10 codes that keep coming up. When the service engine soon light pops on the car goes into a meltdown, stalling, sputtering, then light will go off and all is well until this happens again. Anybody seen this happen before? I can clear the codes, but when the car throws a fit all the codes come back. Codes are 16, 21, 26, 31, 73, 74, 99 and so on. Please help. Thanks

16 is battery overcharge look at the alt output situation first.

Thanks for the reply Maxwedge. I went out this morning to check charging system. I started the car, ses light was on , car ran real crappy, then all of a sudden light went off, and engine smoothed out. So while I was on my way in the shop to get meter, engine just took off like someone had their foot in the gas pedal to the floor. I had to run I mean run back to the car, and shut off engine. Went got my meter, restarted engine, it ran fine, and system was showing 14.0 to 14.2 volts.