Mobo: ASUS P7P55D GPU: Palit GeForce 9800GT PSU: Antec 500W EarthWatts
This is an ancient, well used computer. My primary OS is Ubuntu although WinXP still clings on for some old games.
In the last year or two, I have observed that sometimes when I shutdown (from Ubuntu usually, but it has happened under WinXP), the computer would apparently power-off as normal - all fans stopped, all lights off - and then start booting again after ~5 seconds.
The frequency of this issue seems to have increased; initially I thought it was likely user-error, where I might've selected reboot rather than shutdown. It's not that.
I've checked the BIOS settings; Wake on LAN is disabled. It's set to stay off after power loss. I've not changed any peripherals. I also reset the BIOS completely and replaced the battery.
The "interesting" thing is, last year I replaced my Ubuntu 16.04 install with Ubuntu 18.04 (fresh install, not update). And this time, I did not install the nVidia drivers as they seemed to be causing me problems with suspend/hibernate, and stuck I with the default Nouveau drivers.
So after this OS change, the powering back on after shutdown problem went away!
However, whilst I've been able to put up with the performance hit of Nouveau, I've found it causes Gnome GDM to hang horribly in ways that are hard to recover from (I can SSH in, but I can't restart or even stop the desktop) and I end up having to use Alt-sysreq-REISUB to resrtart.
As a result, I've just caved in and installed nVidia drivers (340.107)... and the restart after shutdown problem has returned.
My guess at what is causing this is a hardware fault that is triggered by the nVidia drivers.
So my question is, what is it likely to be? I'm guessing either a PSU fault or GPU card fault... but it's odd whatever it is.
The main observable difference with running nVidia instead of Nouveau is that with Nouveau, the GPU fan is idling when I shutdown, and then it switches off when the computer powers off, whereas with nVidia, (presumably) as the drivers are unloaded prior to shutdown, the fan spins at full-speed just prior to the computer powering off. Could the fan be failing and causing this?