RE: No restart, optiplex 755 Small Form Factor Computer
It seems the issue is with the system BIOS and it is not able to interact properly with the hardware maybe due to any external peripheral. I would suggest you to remove all the external components connected with the system and check if the issue still persists.
If it persists, then the issue is with the motherboard and it is needed to be replaced.
Keep me posted with the results. I will be glad to assist you.