@Aaron:

try reading into Magic SYSRQ key: http://en.wikipedia.org/wiki/Magic_SysRq_key

next time when X seems to freeze - first do a Magic SYSRQ + R, then try to
imitate a "secure attention key" 
== Magic SYSRQ + k

whether that unlocks it

or if X doesn't show any change anymore but the system still seems to be alive:

Magic SYSRQ Key + r

then Magic SYSRQ Key + w
(what tasks or kernel threads, etc. is stuck)

then the rest of "REISUB" (Magic SYSRQ Key + e, i, s, u, b)

after the next start you can look through the contents of the kernel messages
in

/var/log/kern.log

via

tail -n 3500 /var/log/kern.log | less

(keyboard cursor up & down, quit via "q")

if the messages are too much 

try via

tail -n 7500 /var/log/kern.log | less

or higher

sudo may be needed, if not using root privileges

logging may be disabled, so it first might have to be enabled 

hope this helps ...