Gas gauge problem

[Just me]

Whats up all. I need your help with an anoying problem. Once a year for a whole week, my gas gauge descides to not work at all. As a matter of fact, the needle goes the opposite way(maybe shorting) when I turn on the ignition. Again,this happens to me for about a week and then it'll work again perfectly for another year. I've owned the car for 7 years and its done this for 5. I have changed my sending unit and checked all the rear wiring to no avail. This drives me nuts! I have to keep the tank full cause I don't know my mpg's. Can anyone help? Thanks.