When I click on Shutdown, my PC then reboots and starts up again.
This happens every time.
Windows Support says the problem lies with the Motherboard and the BIOS.
That the BIOS has to be changed in some way.

Gigabyte- eSupport (the manufactures of the Motherboard) have suggested the retailer repairs it. How they could fix the hardware like this I have no idea.
They also are suggesting that there is a conflict between their Motherboards and power supplies.

So hopefully somebody has experienced the same issue, and has found a solution please?

There is one way that the BIOS can change without doing anything and that's if the battery dies.

 

This would only happen if it's a pretty old computer as the batteries last for a long time.

 

The simple test for this is - has the clock (date and time) reset itself at any point? If not then it's not the battery and you can rule that out....

Check for stuff like "wake on lan", "wake on usb" or "power on after power loss" in the bios and disable it, might solve your problem.

Try to press the on/off button for a short time (just press and release it, don't hold it down).

That will shut down the computer regularly (not crash).

Maybe that "solves" your problem.

 

Otherwise you could spend time with a new installation, updated BIOS, and do a lot of testing. Maybe many hours and maybe without result. For problems like this there is no easy "do this and it will work" solution.

 

You might also think about since when this problem exist. Maybe you remember that just happend after you did X. Then try to reverse or change X.

14 hours ago, OneMoreFarang said:

Try to press the on/off button for a short time (just press and release it, don't hold it down).

That will shut down the computer regularly (not crash).

Not the best thing to do. Computers do not like being interrupted and this may cause further issues.

 

This restarting issue may have been caused by a Windows update not installing correctly or was interrupted whilst installing. 

 

My first advice would be to disable automatic restart. To do this you have to run Windows in safe mode and uncheck "Automatically restart". I'm not great at explaining myself so here's a web site that details what has to be done, it also has other workarounds if this does not work.

https://www.partitionwizard.com/partitionmagic/windows-10-reboot-loop.html

 

3 minutes ago, Kron said:

Not the best thing to do. Computers do not like being interrupted and this may cause further issues.

 

This restarting issue may have been caused by a Windows update not installing correctly or was interrupted whilst installing. 

 

My first advice would be to disable automatic restart. To do this you have to run Windows in safe mode and uncheck "Automatically restart". I'm not great at explaining myself so here's a web site that details what has to be done, it also has other workarounds if this does not work.

https://www.partitionwizard.com/partitionmagic/windows-10-reboot-loop.html

 

I had this same problem and as Kron suggests, it turned out to be an incompletely installed Windows update.  I uninstalled the update which was a chore since because it was incomplete, it didn't want to uninstall properly but I had a friend who knows more than I help and got it done.  After properly reinstalling the update, everything returned to normal.

38 minutes ago, Kron said:

15 hours ago, OneMoreFarang said:

Try to press the on/off button for a short time (just press and release it, don't hold it down).

That will shut down the computer regularly (not crash).

Not the best thing to do. Computers do not like being interrupted and this may cause further issues.

You are right, updates should not be interrupted.

But in this case it sounds like the computer restarts every time, always. And the OP didn't mention any current updates.

Shutting it down with the button (short press, not long press) is no problem, the PC shuts down properly.