Everything you described suggests the charging system is not working correctly. Since the 1987 model year, GM has had a very poor generator design that is prone to failure when the battery gets to be more than about two years old. Due to their design, these generators develop huge voltage spikes that can damage the generator's internal diodes and voltage regulator, and interfere with computer sensor signals.
Your first step is to measure the battery voltage. With the engine not running you will find 12.6 volts if the battery is good and fully-charged. Next, with the engine running, you must find between 13.75 and 14.75 volts. If that is low, suspect a failed generator. If the voltage is within that range, it only means it is okay to perform the second half of the test, but that requires a professional load tester. That will measure full-load output current and "ripple" voltage. The most common generator for your car is rated at 90 amps. If one of the six internal diodes has failed, you will lose exactly two thirds of the generator's capacity and ripple voltage will be very high. 30 amps is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
The battery is the key component in damping and absorbing those harmful voltage spikes but as they age and the lead flakes off the plates, they lose their ability to do that. Any time you replace an '87 or newer GM generator, always replace the battery at the same time to reduce the high number of repeat failures, unless it is less than about two years old.
Sunday, May 1st, 2016 AT 9:13 PM