Fuel level gauge not working in 2002 Boxster S

[espooner]

2002 Boxster S. While at Watkins Glenn Intl. last weekend, my gas gauge started acting up. It read "full" all day, when normally it is on empty by the end of the day. Turning car off and on had no effect. Filled the tank that evening (14 gallons). By the end of the first run the next day, it read 3/4 full, and was working normally. 2 hours later, it reads "full" again, and has done so since then, even after driving 225 miles back home.

My Bentley repair manual states that the fuel level sensor and fuel pump are all in one, and cannot be replaced separately. The part alone is $372 from Pelican Parts.

Before sending for one, is there any testing to do to CONFIRM that it is a bad sensor, rather than a bad gauge? It would seem that as the gauge goes full scale, and at least once read 3/4 full, that it is not the problem. Should I smack the pump/sensor with a rubber mallet to free a stuck float? Anyone have any similar experience?
All replies welcome. Eric Spooner