Ford: incorrect gas gauge, gas gauge, ford f150

Question
i have a 1987 ford f150, i have had it for about 2 weeks now and have found that my gas gauge will not stay in one spot it moves when i go up and down a hill and when i stop it goes streght to empty when i put gas in the gauge goes up to show that i have gas but doesn't stay there very long.i have run out of gas 5 times already so i would really like any insite you can give me on my problem.

thanks tim,  

Answer
tim you goingto have too remove the tank and take a look at the sending unit if it is brass your going to have to replace it because what happens is that they get a pin hole in them, and they get gas in them and become heavy and throughs off the guage.