Ubuntu server keeps shutting down after 15 minutes

I have Ubuntu server 24.04 running on a desktop machine. It has a GUI installed (though I don't know if that is relevant). I am running this server as a headless machine at the moment, though until recently it was running with a monitor attached and everything was OK. Now, when I SSH to the machine it will shutdown after 15 minutes, even if I am doing something on it, or if I am doing a long download. I have edited /etc/systemd/logind.conf as follows: IdleAction=ignore HandleSuspendKey=ignore HandleHibernateKey=ignore HandleLidSwitch=ignore HandleLidSwitchExternalPower=ignore HandleLidSwitchDocked=ignore Everything else is commented out. This is a Desktop machine so issues of lid closure are not relevant even though I have specified them (copied from another post). I have also run sudo systemctl restart systemd-logind, followed by a reboot. I can RDP to the GUI using Remmina, and the only power options available are to set Power Mode to 'Balanced' (the default), and I have set 'Screen Blank' to 'Never', though I would have thought this is not relevant. I am at a loss as to what else I can do. Any suggestions, please. As I said, this all seemed to start when I went headless! Regards, StuartM