Car voltage problem? 3 of 4 load tests ok. Vehicle dies frequently.

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I have a 2001 Oldsmobile Alero that is having electrical issues. Aftermarket radio turns on off intermittently with ignition key turned in on position and when car is started and turned off. It was installed by a friend of a friend. Had alternator and battery load tests done 3 times now, twice by my repair shop and once by my friend who works for auto zone and both check out ok. My vehicle has died 4 times since having issues and have had to have my car jumped each time. I had my aftermarket alpine stereo taken out checked by a radio shop to check if it was installed properly. Radio was removed within 2 minutes the load test they conducted failed 11.5 rest and a 6.5V drop. I've been told I should get battery checked at Interstate battery center. Interstate battery that is in the car I have been told is only 1 year old from previous owners who I spoke with. My repair shop pulled up 27 error codes in computer after security light had come on. A few of the codes are U1025 Loss of serial communications for class 2 devices. B1328 vehicle system voltage is greater than 15.9 volts. U1000 class 2 communication malfunction. My repair shop has replaced the ignition switch which I was told help get rid of 4 DTC codes and I had to have the ac compressor assembly with drier repaired recently. I have been told to leave the radio out for 3 days by my repair shop. They are checking and clearing out the codes and I have to bring it back to have it checked to see if the codes come back. I have no idea what to do and feel I am at a loss with this vehicle. I notice when driving the vehicle that pressing on any electronic buttons in the car as well as accelerating in and from idle position the car interior lights dim and I can hear a change in voltage from low / high, high / low it fluctuates. I've already wasted enough money into the car itself. I have read many forums of vehicle owners replacing alternator and battery as well as the starter in varies cars and it does not help the issue. I do know I have some sort of voltage issue going on but that all I know. Any suggestions on what could be causing this problem along with the DTC error codes? Thank you!