Funky voltage
 

At work yesterday I turned my jeep on and noticed that the check gauges light is on, and the voltage was dropped all the way down. I tried it a few times when I turned her on goes up to around 12 and then drops. On my drive home, about 30 miles the gauge was down the whole time. All this I have read on many posts and it seems the answer is always the Alternator.
So on the way home I stop in town at NAPA and grab the 90amp alternator they had in stock. When I get back in the jeep and turn it on voltage goes up to 14 like normal and stays. It has stayed like that for multiple starts now.
I couldn't find any others saying it went back to normal, any idea what could be going on? is it safe for me to return the alternator and save some money?

Just replace the alternator. I did that song and dance before and it was the alternator. The brushes are worn out and making poor contact. Sometimes they will work and sometimes they won't.