A technician is troubleshooting a workstation that randomly reboots and locks up under load, such as when rendering video. The system has been running fine at idle. The RAM passed an extended memory test, temperatures are normal, and the storage drives report healthy SMART status. When the technician connects a multimeter and PSU tester, they notice the +12V rail readings fluctuate and briefly drop below tolerance during heavy CPU and GPU activity. What is the MOST likely cause?