They are very straightforward, but repair might not be easy or possible.

First remove the sending unit and examine the windings. It is a simple affair with a resistor dragging across resistor windings. If the windings get corroded or worn-out in spot the gauge becomes unreliable.

I'd guess that you've either got corrosion (cleanable) or the wires are worn through (bad).

It's definitely worth looking at.

Also, check the unit with a multimeter (ohms) when you have it out. Others BB members will give you the proper ohm values that the sender should return when at the top or bottom.

I had to rebuild the sender in my sedan (it was amazingly corroded) but a very simple fix.

I also repaired the sender in my 940 and that's now part of the 700/900 FAQ.
http://www.brickboard.com/FAQ/700-900/FuelSystem.htm#940FuelSenderRepair

Be cautious, but don't be afraid of the gas.