The idle raises on its own to 2,500 RPMs?

1989 Pontiac Grand Am 2.5 Liter mileage: 120,000. I have been working on this car on and off for one month. This is what I have done: Symptoms: The car intermittingly raises the idle on its own to 2500 rpm. I have code scanned and code 35 comes up. This means a problem with the IAC control valve. I have replaced the IAC and the throttle position sensor and the 02 sensor and the code still comes up. (Service soon light on). I have recently invested in an Actron Tech Tool, which has been helpful. I have replaced the battery because it was leaking acid at the terminal connections which I thought could cause the computer to not function correctly, hence problem solved. The new battery did not solve the problem. (I did clean all the acid from the terminals) I have checked the battery when the car is running with the tech tool and the voltage is dropping to 6 to 7 volts on the tech tool screen. I checked the voltage from the battery to the ECM and it is 12.5 volts on both orange wires running to the ECM. As soon as the car drops to the 6 volts the IAC reading goes up to 255 on the tech tool. Then all by itself the car will start to idle normal at 1025 RPM and the voltage will read 13.5 volts (running). At this point, the IAC will be at 28. (Which I believe is normal). I will shut the car off and start it again and the voltage on the tech tool will be back at 6 volts. I think that the ECM itself is intermittingly showing the wrong voltage, which is causing the IAC to go out of control limits, and causing the idle to rise to 2500 RPM. I even double-checked over and over to make sure that the voltage going into the ECM is staying at 12.6 volts. Please help me. I am thinking of getting another ECM but they are expensive. I would like to hear from you before I make this purchase.