I sugest you to install/enable sysstat if you have not done that already, and with those metrics you will have some great starting point about what resources may be the culprit next time it happens. It will help you pinpoint if there is a hardware related issue.

Do you have kdump enabled? If so, you can try to force a coredump when the system freezes, so you can uater analyze what the issue is. It is harder to follow this path, as you may need analyzing such dump, but it will help you identify issues not only on the hardware side, but on the software side as well.

Both tools are our bread and butter for RCAs/postmortems