308 Fuel Gauge Problem...?

My 1981 308 suddenly developed a problem with the Fuel Gauge. When I fill the tank,(and Always stop after it's Clicked off at Full-never overfilling) The Gauge works properly until it gets down between 1/2 and about 2/3. Then,it starts spazzing out,and jumps down all the way to Empty. Sometimes it will flip back up,but then down again. But,mostly just stays down at Empty. When I make Right turns,or drive on a Right Cloverleaf on an Expressway "On Ramp",it goes back up. But once I drive straight,it will go back down again and stay there.

Then,when I Top it Off,it will go back up to Full,and work properly until it gets back down in the 1/2 to 2/3 Range.

I'd like to fix it myself. I have some Mechanical ability. But,was just wondering if anyone else had this same experience,and/or may have a suggestion as to what the Problem could be....Is it the Gauge,or the Sending Unit in the Tank/s? Where might those be? I'm thinking that the Tanks are a real Pain to get off and into. Thanks for any help!!!!