Strange things with my Impala 3.4

Hi guys. Sorry i was to busy last month. Like i mentioned before i still have my same problems(cat symptoms), major loss of power and hard shifting. I think nobody gonna belive what i have right now. Alredy 2-3 month i do not have any SES light. I was driving 100 miles per day. I have 97000, 3.4,2003.
Not long ago and today i notice this: when i have 1/8 or even less gas left in my tank, my problems is dissapearing. I have a car same like i bought it. Everithing seems to me fine. I think this is only isue i have to figure out. Million times i chekd my fuel pressure and it was fine. Mabe its fuel flow or something bad with fuel delivery?
What you think guys? You can find a lot of my posts on the same problem, and i think this is it.
Any help apreciated.
Thanks
STEVE

I have Alldata, so i think its not gonna be a problem to change something. But i dont know what. Can it be vent pipe, or restriction on feed and/or return pipe(i mean pressure is good, but maybe flow is bad).

I would inspect the tank vent system.

Ok. I am gonna take a look on satturday. Thankyou for advise. I gues you dont have a picture. If you have something it would be helpfull.
Thanks again.

Somebody can show me picture, or explain how to fix that? Or simply where is that. Cause ALLDATA have only thisFuel Tank Vent Valve.....10439027.

$28.57

BTW forgot to tell sometimes after long trips, i hear some buzzing from the back (looks like pump) What can cause that?