It seems I am having the same problem though on a slightly different hardware (Intel graphics) and software (LXDE/Openbox on Ubuntu 12.04.2) but also dual-head setup (2 monitors). There is a bug report about xlock occasionally dying on dual-head setups that might be related. More importantly, though, linked in a comment to this bug MMlosh points out that xlockmore has been removed from unstable. It is not available anymore in recent versions of Ubuntu.
I suggest using alternatives such as xscreensaver or slock (which is part of the suckless-tools package).