Drop the fuel tank and find out. It only takes about an hour with basic tools. Change out the fuel filter (inline) and check the strainer on the pickup. Look for rust in your tank, and if the car was sitting for a while and you didn't dump the gas out, you should probably dump it out and put fresh gas in. Maybe you've got a loose baffle, or maybe someone dropped something in the tank? Maybe the sender unit is also corroded and needs to be replaced.... Like i said, you'd have to drop that tank to figure out anything.