Today morning I made some other tests and I'm convinced that tho problem depends by NavData. In particular, by SIDs. Infact, I tried to introducing the route, and then the STAR for LIRF (before SID for LIMC) and all was ok. Introducing SID FARAK5P for LIMC I noticed some wrong waypoints, e.g. (INT) that were too far from the route. The FMS shown me a distance from nearest waypoint -12354647474. Deleting that waypoints and EXEC the FP, everythings came back correct.