Why my page one ranking keep disappearing from google and then reappearing a couple days after?

I've noticed that page rankings can change from day to day and even hour to hour. I think it depends on the newest sites and how often Google crawls your page. If it hasn't done it yet that day it might not rank high enough for page 1. Or maybe your traffic was lower that day so Google didn't rank it as high. I would start using Google ytics and checking the days your traffic is lower and comparing that to the days your page rank drops. Hope that helps!