cabal.netlify.com

Hello,
Sorry for the long message, but I'm hoping that the detail will help someone who has the expertise to see what the issue most likely is and let me know so I can take care of it.
Over the past 3 years as a serious hobby I've built several DIY PC's and rebuilt several laptops and several dozen off-lease PC's, but haven't seen this problem before so am looking for any help and advice from more experienced members of the community who might know what this is and what I should do to fix it..
I bought a used HP 8200 Elite SFF PC (Product Number: XL510AV) with a Windows 7 Pro COA, hooked it up to a KB/M and Monitor, turned it on and it booted to BIOS fine.
I then inserted a Windows 7 Pro full version CD to install the OS on a wiped and formatted hard drive and the initial phase of the install went fine. It was missing the network/ethernet driver so I went to another PC to get the 8200 Elite's driver for this model from the HP website and copied it to the 8200's desktop from a USB stick, then installed it, and it connected to the internet via the ethernet cable fine and downloaded a bunch of Windows updates.
I went ahead and installed the Windows updates, which all seemed to install fine and finished with the standard 'reboot' request - which I went ahead and did like I've done many times before.
The 8200 shut down as expected, but when it started to reboot, instead of seeing the expected HP startup screen the screen was blank and the PSU fan started running faster/louder and faster/louder - so I pushed/held the power button and did a hard shutdown.
I thought the PSU might be bad and/or one or more RAM sticks might be bad, so I disconnected everything, took out the motehrboard battery, replaced the PSU and the RAM, replaced the motherboard battery, reconnected all the internal cables, hooked the 8200 back up to the KB/M & monitor & power cable and pushed the power button.
The PC then booted to Windows fine, but I did have to reinstall the HP (Intel) network/ethernet driver, then ran Windows update again. I went ahead and typed in the Windows COA and activated it at this point (and it activated just fine as expected), then I went ahead and installed the Windows updates that downloaded, which again all seemed to install fine and finished with the standard 'reboot' request - which I went ahead and did.
The 8200 shut down again as expected, but, same as before, instead of seeing the expected HP startup screen the screen was blank and the PSU fan once again started running faster/louder and faster/louder - so again I pushed/held the power button and did a hard shutdown.
I disconnected the external calbes and also every power and data cable internally, took out the RAM took out the motherboard battery again, pushed/held the power button to drain residual power, replaced the motherboard battery and tried resetting the CMOS (not sure why, I just tried it..pushed the CMOS button for at least 5 seconds as advised on the HP web site), then reconnected everything and was going to boot again, but this time I noticed that the front panel power light on the front came on when I connected the power cord into the PSU at the rear of the PC and the same thing happened - the screen was blank and the PSU fan started running faster/louder and faster/louder - so I pushed/held the power button and did a hard shutdown.
At this point I'm not sure what to look at or do next.
Has anyone seen that type of problem before caused by things other than a bad PSU or RAM sticks?
Could it be a bad motherboard? What else can I do to diagnose that?
Could it be a power button or wiring intermittent short?
What else have any of you seen that could cause this sort of thing?
I thought about doing a complete disassemble all the way to a bare case to check every wire, cable, connection and screw; and then reassemble and try again; but if it's obvious to someone who's seen this before that it's a bad motherboard (and/or something(s) else?) then I'd like to save the time/trouble and just get a replacement HP 8200 SFF motherboard (and/or whatever) and install it.
Thanks for your thoughts and your help.