Thank you for your advice. I applied it in full.
After restarting the computer, the memory is 6.88 GB. After launch iMovie: 5.82 GB. Only one project and one
event are available. So I tried again to export the film; result:
1) first attempt: Error -108 after 3 minutes, at which point the memory is 5.28 GO.
2) second attempt : after restarting the computer and relaunch iMovie: iMovie crashed after a
minute.
3) third attempt, after restarting the computer and iMovie : export has been running for a
hour then 108 err! no problem of RAM, the activity monitor shows that iMovie uses
approximately 1.6 GB of RAM in a stable manner, and the RAM is also quite stable around 3 GB.
During attempts, no other application is running, only iMovie.
For me, the -108 error has nothing to do with a problem of memory management;
Please note that this problem is the same on my second computer, the same iMac (late 2009 - i7 - 8GO RAM) running with Snow leopard.
Err 108 isn't a Lion's error.
For me, ther's a bug in iMovie.