I did the zj alternator swap on my 92 xj and I got it all back together it acted the same way my 90 amp alternator did.
I didn't upgrade the wiring just yet
I had the alternator tested and tested good
I know it's the 136 amp alternator
I also have a yellow top optima

I am going off the gauge for most of this which I know isn't super accurate. What I find funny is it will start out running around 13.5-14 and after about 10 minutes of driving I'm down below 12. When it's low on the gauge everything dims when I turn my blinkers on. So if anyone could help me out that would be great thank

 

I would start by making sure that the battery terminals are clean and tight and clean all of the grounding points and if you still have the problem we can go from there.