OK. I got it. Thanks to your precious tips, i tested and tested, and i saw the light! The problem is just related to fuel load: too much fuel load and too much errors, as you said.
I simulated a 99 lap race in Elkhart Lake, with 3 pitstop of course, and 16-18 runners ended the race.
And: i simulated a 95 lap race in SPA (yes, SPA), with 3 pitstop, a 3h 02m race; no problem of any type!
So: if you want to do a very long race, you must set 3 pitstop to reduce the stint, as Roberto and Mr Lee said.
Now i'm ready to have an endurance race: probably track are Bugatti Le Mans, Sebring and Spa, from 2h 20m, to 2h 43m to 3h 02m. Now i need a teammate to drive with...
Thanks to all for your tips and patience...