Dell Plasma TV + Windows 10 disaster HDMI Help

Sorry this is kind of long, but here is the summary if you don't want to read all this:

TL;DR: Windows 10 works with old Dell Plasma Dell W5001C unless I turn off the TV, then it acts like there is no connection. Only works then with VGA. Must completely re-install Windows to get it to work again, until the TV goes off. Motherboard splash screen shows fine.

I doubt I'll find help since this TV is old and this is a complex question but I thought I'd give it a try, since I've spent so much bloody time on this!

My parents have a Dell W5001C Plasma Display that has worked fine for years with their HTPC. They used Windows 7 for years on this tv with absolutely no problems. 

The old PC was an Athlon X2 3800+ based system (Socket AM2+)

I recently upgraded them to a more modern Intel system, LGA 1150 Pentium G3220 based system. I installed Windows 7 again since my dad was used to it. 

No matter what I did I could not get the TV to recognise the board through HDMI. Just the blue floating square you get when there is no connection.

I tried many trouble hooting tips, including "resetting" the TV, trying other connectors and cords, everything I could google or think of.

So instead I abandoned that upgrade and built them an even better PC with a socket FM2 AMD A10 6800k. I tested this first before doing the upgrade, and I could see the motherboard splash screen ! So I was quite happy, I thought the issue was something to do with the Intel based motherboard not playing nice with an old TV.

So, I install Windows 10, all is good, get to the Windows 10 screen, then I turn off the TV to do chores. I come back to set up the system, come back and AGAIN same problem, the TV will not recognise the computer or vise-versa! I tried resetting, unplugging the TV, using different cords, everything.

It's bizarre because I actually loaded Windows fine, full picture, until I turned off the TV!

It loads the motherboard splash screen, shows the Windows loading circle, then goes dead (blue floating square or coloured self-test feature check.)