Petrol indicator

fuel gauge and light showing on empty after standing for a while, when i fill up and and know there is suficient petrol my light keeps going up and down on hills, i have had it into the garage and had 3 tank floats 2 showing full when empty and the 3rd which is a ford on was ok for about a day and has now gone back the same, could you tell me if it could be somthing elce they should be looking for, each time they have had the vehicle for at least 3-4 days,

thank you