Segmentation fault on mongod processes and mongo shell

Since we upgraded from 2.4.10 to 2.6.5, some nodes crash randomly by segmentation fault (mongod.txt). On these nodes, the mongo shell won't start and do a segfault on startup (mongo-shell.txt).
We have tried to recompile mongodb with different versions of gcc or of the gilbc, which don't solve the problem. Same result when we disable all the compilation options (use-system-yaml, use-system-boost, use-system-pcre...).
The default compilation options are available in this ebuild : http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/dev-db/mongodb/mongodb-2.6.5.ebuild?view=markup

Other nodes have the same configuration, and mongo runs perfectly.