What is your EXACT power supply model number? You will find a label somewhere on the PSU itself with the specifications and model listed. 800w, means nothing. There are 800w units out there that can't adequately sustain 450w and even a good 800w unit could have problems. The biggest red flag I see is that there are actually VERY few, if any, 800w units still being made that were of any decent quality at all. That means it's either likely a poor quality unit OR it's very old. In either case, it could absolutely be related to your problem.
It might NOT be, but it certainly COULD be and you shouldn't assume due to a paper specification that the unit is either capable or healthy.
As far as the legacy bios setting is concerned, if you have Windows 7, you MUST have the BIOS configured to CSM enabled and "Other OS" rather than the "Windows 8/10" UEFI setting. Windows 7 is a legacy operating system. Honestly, you need to seriously consider ditching it and upgrading to Windows 10 which is far superior anyhow. Obviously, to do that properly you'd need to do a clean install of Windows so I wouldn't necessarily worry about it right now until you figure out and resolve your current issue, but I would certainly plan to do it at some point.
There are too many reasons to list, not least of which is much better driver support and memory management, besides the fact that come next December, Windows 7 is 100% done anyhow. There will be no further support, not even for critical security updates, after that.
I'll be honest about the motherboard, I think it's a waste of money. If you're going to spend money on a new motherboard, it should be something a lot newer, but then of course you're going to also need a compatible CPU and memory as well. I just can't see recommending the purchase of a motherboard for a platform that was weak to begin with, plus is about 5 or more years old. It's money that would be better spent towards an upgrade, not a sidegrade, but you gotta do what you gotta do I guess.