Random lean a/f during cruise

[4g63tcrazy]

So I installed an AEM UEGO wideband on my daily. Basically just so I could tune it for gas mileage using the SAFC. So on my way home, I'm cruising at about 70mph, the a/f was bouncing between 14.7 and 15.0 with the SAFC at 0%. But I want to save a little gas so I usually have the SAFC set to -4%.

Now as I'm cruising, I start going up a hill so I start approaching 1psi. Then I feel the smallest little studder.. Like if you werent paying attention you probably wouldn't feel it. But I always pay attention to my car as I'm driving. Anyway, every time this tiny studder happened my a/f would go as lean as 17.0 to 20.0! So I would get back into vacuum while cruising and the a/f would just stay that lean.. So I would go into the settings on the SAFC and richen up the mixture til the a/f was in the right spot again. (14.7, 15.0)
Once it get's back into that right spot I tried leaning it out with -4% fuel again. Then the a/f would be right around 15.2 to 15.4 and it would stay like this until I got to about 1psi.. Then I'd feel the tiny studder, look down and the a/f went lean again. Then richen it back up ect..

So I kept playing with it and if I have the SAFC at -2% it will not lean out once I hit 1psi like it did before but the a/f would still be around 14.7 to 15.0 like if it was at 0%.

The only thing I could think of would be the fuel pressure regular not upping the pressure per 1 psi and then the car leans out.. But my car will stay leaned out until I richen it up about +6 percent!

So what do you guys think? I can't think of anything else other than the fuel pressure regulator or maybe the ecu is not liking me taking out fuel.