My gas guage on my 2006 Malibu LS goes from full to empty. Regardless of the amount of gas that I have in the tank. It will go from full to empty then back up to full then back down again. It'll stay up for a little bit sometimes while driving but it does not ever show the correct fuel level. Do I have to replace the whole fuel pump/sending unit? Or what should I check first? Please help....it's been going on since I bought the car 8 months ago