Same car as our firebird... I would say maybe a kinked or clogged brake line - sometimes they swell internally & tear just like when we get a heart attack. There are also abs sensors on the wheels that may have broken wires that can cause intermittent signals. Finally I believe that there is a magnet on the flywheel or the crank shaft - it is called a flywheel trigger - it tells the computer how many rpm's the motor is turning - sort of like a clock pulse. If the magnet falls off or the sensor goes bad then the computer gets bad data and starts spitting out stupid stuff. It's the old "GIGO" scenario... Garbage in = garbage out.
I hope that helps and I am sure that Marc will fill in the blanks or correct me if I am off base.
Well, I believe I found the problem on my Camaro. Shortly after I start to get the ABS inop light randomly coming on. It would reset after turning ignition on and off. A few days later the battery died, after new battery installed car seems fine now.
You fixed part of the problem - a new battery does nothing about the left rotor being chewed up.... I suspect the brake line, when they internally fail then they can act like a check valve ....