It's the material they use to make the fuel level sensor, read something about it in the newspaper a few years back - most American manufacturers use a sensor that gets a specific type of corrosion from some particular additive in the fuel formulation we use in the US. The corrosion makes the sensor malfunction exactly like what yours is doing.
There's a fuel additive you can use every other tank of gas that removes the corrosion, but if you stop using it, the corrosion just comes back. Best to watch your miles and do what Luis does = fill up more often.
It sucks, but the alternative is to buy a more expensive car... is it worth it? Not imo.