There is a problem with the sending units used in these tanks. They are very susceptible to contamination from the sulphur is the formulation of some brands of gasoline. The amount of sulphur fluctuates by brand, region and time of year.
Sulphur contamination of fuel senders acts EXACTLY like the symptoms you are describing: Shows either always full, empty or half.
There are government standards, but sometimes an unusually 'rich' batch of gas gets to the pumps. 2 years ago in Florida, all the Shell/Texaco gas stations were shut down for an entire 3-day weekend due to high sulphur content discovered during random tests. The gas was removed and replaced...a big expense no doubt!
Anyway, The only known effective method to clean the sender is through the use of the chemical polyetheramine. This is the main active ingrediant in Chevron's 'Techron' fuel cleaning additive found in thier gasoline. The same chemical (PEA) is also leasd and used by Shell in all thier grades of gas, which Shell calls 'V-Power'.
The concentrations found in pump gas are not strong enough to effect a change in a heavily coated sender, so go to the store (Wal-Mart, maybe) and get TWO bottles of 'Techron' fuel additive and put both of them in with a full tank of gas and do not add more gas until almost empty. If an exceptionally large fuel tank (over 22 gallons) use 3 bottles.
You will be surprised that it works...but it does!