Random system reboots when your rig is peaking its performance (when it uses most power and is effectively squeezing all the juice out of the PSU).
I have never heard of anything catastrophic happening due to low voltage. The device just stops performing a task it was performing, and your platform, detecting this as a serious error (paranoid as it is), reboots your machine to save your hardware.
I am, however, using a 325W PSU for a computer that really requires 337W... it does reboot occationally. Since you deviate more than I do (you lack 19W), and particularly if your PSU is a cheap one that is usually shipped with pre-built computers, then I definitely recommend that you buy a better PSU.