The problem is often 100% memory related, but not always, if you have done all of the other suggested things to correct this (IE Don't use reset button, backup BIOS matches main, CPU PLL Overvoltage disabled, ect)
The issue is caused when memory errors at bootup cause the ME firmware to mess up, thus a reboot cycle is invoked until it's corrected (IE hard reboot or warm reboot from the BIOS)
How many instances of HCI memtest did you run, with how much memory in each when it reached 600%?
Ahh yes, please do update the backup BIOS, you should have done that first and long ago!
What I meant by not being the same issue as discussed here was your mentioned issue was with sleep, so I just meant you should create a thread and ask for help with sleep issues as these issues mainly deal with cold starts not sleep as that is it's own beast
To resume from sleep you have to be 100% tested stable, with several stability testing tools, and still may sometimes need a little bit more memory voltage than you need for stability, in order for S3 sleep to work. Of course it also must be actually working in the BIOS version you are using too, sometimes older BIOSes it is broken until they fix it. I am not sure if it's broke on your model's latest BIOS or not still, I haven't ran into enough users of that board to know for sure.
I see you mention you have the same issue on cold boots, sleep aside, that is relevant to this thread as you mentioned! So for now, do all the things I suggested and don't use sleep for a while, that way you can rule out sleep as part of the issue