Ghost in the Machine

[Geos54]

My Dell Optiplex 9020 has developed a mind of its own. When I Shutdown it will only Restart. The only way to Shutdown is to pull the plug. Everything was fine until I tried to create a Win11/Linux Mint dual boot system. I mothballed this a few years ago so there was no OS to start. When I installed Mint on the it worked fine. When I tried to install W11 with a USB drive created with Rufus I got a BIOS message about EUFI and changing to MBR instead of GPT. I pressed Enter and it shut down. When I pressed the Power button a few times nothing happened. After about a half hour I tried again and Mint did boot up. Everything seems to work properly but it won't Shutdown, only Reboot!!! Any ideas???