I forgot to tell that the system was built from ports. I'm posting here my /etc/make.conf, perharps the gcc flags may be of your interest.
CFLAGS=-O2 -pipe -march=prescott -funroll-loops
# added by use.perl 2007-06-22 23:29:22
When it started to fail, I was using more flags in the CFLAGS variable. As soon as I thought that could the problem, I started rebuilding the whole system with this options. In both cases it happened.
And well, testing with 9631 module and libs from the port system rendered the same behaviour. It took more time, but finally it happened again. My Xorg.0.log.old shows this :
(EE) NVIDIA(0): Failed to attach to shared memory segment 14549034.
Nothing special needed to trigger the problem. In this case beryl was using one core. I was able to connect from another system and kill it. Do you want me to try anything special ? May be compiling without march or such ?
Tried to change options in beryl but didn't helped (reading the forum I found may people telling to change the render path, etc ... Tried all combinations without success).