Definately something else going on there...
I would definately drop the tank and clean it out..If your not comfortable with doing such a job as this...get a shop to do it.There has got to be debris of some sort in your tank to do this....filters should last much longer.If it's rust from the tank itself,then you might want to get a new or used tank...Also try a different gas station??
If you do this yourself,BE CAREFUL!You are working with gasoline after all...Even when empty,the vapors can ignite...
Good luck-Milt