Sounds like the motherboard. Also a chance it could be the PSU, but this is less likely.
Are you running at stock settings?
As an aside, resetting the BIOS with the jumpers isn't "flashing" the BIOS. Flashing the BIOS is where you download a BIOS file from the manufacturer's web site, and use a BIOS flash utility (which can be part of the existing BIOS, or run from a bootable thumbdrive) to update the motherboard's BIOS code. What you're doing with the jumper (or by removing the battery) is called a "CMOS reset".
The years just pass like trains. I wave, but they don't slow down.
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