Fuel Gage Erratic

Here is this week''s challange.  1986 Devill, V8, with front drive.  The fuel gage is sometimes erratic.  It will read 4 gal when there is 11 in the tank.  As the car is driven the gage gradually moves to the correct level of fuel indicated.  Any ideas?  Thanks for the help.
 
Craig 

Sender likely cause, you can test it when its out of car with ohmmeter. Fuel gage has buffer built into circuit to keep gage from moving too fast as fuel sloshes in tank while driving.