Occasional failure to boot up first time

hsehestedt said:

Steve makes a good point. You might try disabling one at a time though so that you can really isolate whether one in particular is causing you difficulties while the other may not.

I was paying attention only to Windows Fast Startup because the manner in which it operates is simply more prone to causing potential issues.

In case you are interested in knowing a little bit more about what Fast Startup actually does, here is an article that does a pretty good job explaining how it works:

The Pros and Cons of Windows 10’s “Fast Startup” Mode (howtogeek.com)

As the problem still arises with both Fast Startups disabled I guess I need to get the PSU and capacitors checked out - any particularly good place to get this done, as it's beyond me?

Ok, after all the advice I finally decided to take the PC to a local repair shop to check it out. They updated the BIOS, but could never get the same problem. When I brought it home I almost instantly hit the same problem. I watched my wife start up the PC (she said she had never had any problem), and noticed she tapped the Power button lightly and let it go. I always held it in for half a second or so until I saw the PC spring into life. Since I started just lightly and quickly pressing the button and releasing I have had no problems (so far!). Famous last words but it looks like I have been the problem. I would like a more technical understanding as I really don't understand this, but it seems to be true as time passes?

I was going to ask if the Power button was getting stuck.

No, don't think it is sticking, just being held in too long, presumably upsetting some internal eletronic thing

A half second or so should really make no difference. However, holding the power button down for 4 seconds would cause a forced power off.