Fuel gauge acts weird

hello, recently i got an old lesabre, that i had to replace the fuel tank, sender, and pump. also the lines and brake lines. anyway when i gas up my vehicle the gauge goes past full, by about an 1/8 of an inch. when it starts dropping below that, it will stay at a certain level while stopped, and as i start moving, the gauge will move forward more towards full. what is causing this? the car runs great. also i have notices a few wires going to the tank, located at a section split in the very front section of the trunk. is this where i check for a bad ground? if i had a bad ground would my fuel pump still work?

You can learn to live with it, or replace it! (The sending unit) This post may help;
http://www.automotiveforums.com/vbulletin/showthread.php?t=132711&highlight=fuel+gage+sending+unit

i just dont understand how a brand new sending unit can be worn out. it did this since i installed it. is it possible this is a bad ground???