Dell OptiPlex 990 Restarts During OS Boot/Installation

Hello Dell Community,

I am facing an issue with my Dell OptiPlex 990. The system restarts automatically as soon as the operating system starts to load. I also tried installing a fresh OS, but the PC restarts even while the installation files are still loading.

Here’s what I have already tried:

Changed the processor and memory modules

Reset BIOS to defaults

Attempted different operating systems

Swapped the power supply

Ran the Pre-boot System Assessment (ePSA) test → no errors were found

Unfortunately, the problem still persists.

Has anyone experienced this before, or could guide me on what else I should check (e.g., motherboard, overheating, BIOS update, etc.)?

Thanks in advance for your support!