Difficult dad (family dynamic)

Coming from a dysfunctional family, I have always tried to stay positive and make the best of it. As a kid both my parents drank regularly. With age my mom has cut way back, my 70 year old father still drinks a solid 6 pack a day at least...

The past 4 years he's been on an infusion for melanoma in his lymph nodes...he's become more and more irritable as time goes on...picking fights nearly every time I see him over things that make no sense. He circles back to the same points of contention which always have to do with my mom.....the famous one is, she made 26 lbs of meat for Thanksgiving like 5 or 6 years ago..

I live an hr and a half away and my visits are not often, I mostly come to see my neices. This dynamic bums me out, I can't wrap my head around why he wants to spend whatever time he has left being so argumentative and angry.

I also can't understand if this is from alcohol, the medication infusion (which he's been off of 6 months), potential pain/side effects from the medication, possibly memory/cognitive loss, or if this is just who he is.

I've tried arguing back. I've tried to descalate by saying "I don't come all this way to fight with you." I've tried putting down all the eggshells to avoid these situations...please commiserate with me or leave your advice if you have any