1997 Nissan Altima Battery or Alternator or Anything else?

Hello, everyone.
I really would like to know your opinion about the problem I am having.
Let me explain.

about 2 months ago, while I was driving 97 nissan altima on the highway about 70mph, my radio sound got weird and volume was kind gotten lower by itself. I thought that the station I was listening had some problems. However, it turned out that all the channel sound weirds. That happened for about 5 minues. And then radio sound came back fine. And suddenly, AC does not blow cold air any more and also blower did not blow much air out. Luckily I could get home.
Next day, I call AAA battery service, and guy checked battery and alternator. He tole me that alternator does not look good, but, machine showed that battery is bad. So, I went to Autozone and replace the battery right away. And It seemed like that all the problem had been taken care of for about 2 wks.
Unluckily, it happened again while I was driving on Highway. This time, while I was driving local, suddenly airbag light came on, and wiper was very slowly moving. I hope I can make to home. When I got on highway, the speedometer and tacometer were dancing, and several other lights were blinking. It was total mess. However, car ran fine. However, after driving like that for 10 min, car was getting sluggish and finally, engine stopped and I had to pull over and towed the car.
I thought that the problem is due to alternator. Anyway, my neighbor checked the battery, and bettery was totally drained. So we thought that alternator is defective. He charged my battery, and I went to Autozone to check the alternator. Voila! The test result was battery and alternator was fine. No problem at all.
They told me that it can be loose ground and etc.
My neighbor checked alternator, and ground on alternator was little bit loose, and the original nuts(?) For ground on alternator was craked, so that he put the ground wire to the right next nuts on alternator. And I did not have any problem for 3 wks. And it happen again yesterday. The symptom was same, radio made wabbling sound. And tacometer moved back and forth with turn signal. So I went to O Reiley this time. They told me that battery level is too low around 15% and they can not check the alternator due to that reason.
So I went to autozone again. They just checked the battery, and told me that it is fine. And To check the alternator, they want me to start the car, and car was completely dead. I could not understand how the battery result was fine.
Anyway, they rechared the battery for 40 min. And I came back home.
Today, I went another O'Reiley, and the result was alternator is underpowered. It was about 30A (?) Lower than normal range. And I went to autozone, and ofcouse, the results were fine as always.
They both used same kind of handheld tester.
And I went to another O Reiley, just in case, and battery and alternator were fine. When I went to autozone today around 2pm, they told me that battery level was about 85%. And when I visit O reiley around 6pm, the level was 75%. To me, it looks like that alternator does not charge the battery. I checked at home the battery voltage was about 12.5 without engine on, and with engine on, it was about 13.7.
With a lot of load such as light, ac, . It was about 13.4.
All the connection looks fine, and the ground wire looked fine. I don't know how to check ohm, I could not do that part. I have multitester. If someone can let me know how to check ground wiring, please let me know.
Long story short, I'd like to know why this happening.

1. Short?
2. Loose connection?
3. Alternator?

By the way, battery is 2 months old from Autozone, and Alternator is 11 months old from autozone.

Thank you very much for reading my long, poor writing. I really appreciate your help.