David- The F355 (2.7 and 5.2) rollover valve sits atop the fuel vapor separator cannister (disareator) and is called the anti-flowback valve in the parts catalog. It is number 21 in this drawing from Ricambi's online parts catalog and is part of the antievaporation system (evaporative control system). If you have the WSM it is shown on page 220 (5.2, 146 for 2.7) of Volume 1 and there is a description of the rollover valve on page 222. It is a simple check ball that shuts off fuel flow if the car is inverted, or even on a very steep slope.
If you do not have an F355 WSM, go to ferraridatabase.com and download one.
Looks like there are all kinds of little valves in there that could be clogged.