The power supply unit (PSU) is the most likely culprit for intermittent shutdowns when there have been no reported changes or symptoms related to software or overheating. The PSU could be failing to provide consistent power due to faults within the unit itself or due to inconsistencies in the incoming power from the power source — such as sags, surges or brownouts. Temperature-related issues, like CPU or overall system overheating, can cause similar problems but are typically accompanied by other symptoms such as performance throttling or error messages related to heat. Other components such as the motherboard or RAM usually exhibit different signs of failure, like POST beeps or memory errors, not typically causing the system to shutdown intermittently without other warnings.