Also check the MCLKSTS when this happens, I doubt that you are having a clock failure but if you are turning off the oscillator that the device is running and the NMI is disabled then it could generate a reset. This, WDOG and external reset input are the only possible reset sources.
Also can you let us know what the application is trying to do, toggling an IO, configuring PLL and reconfiguring it in a loop., writing to WDOG registers..?
Best Regards
Santosh