Please see this post, and try the suggested 4+7, or you can also short 6+7, both of those would cause instant reboot and then initiate booting from the backup BIOS (Remove short after first/initial reboot)
How to fix "dead" dual BIOS motherboard if flashing failed
You can also try the auto recovery trick mentioned at the end of post #2 here
BIOS Flashing - A "How To ~ Qflash Guide"
But, it sounds like you may not need to do any of that though, since you can access the BIOS sometimes, that means the BIOS is OK. You may just need to do an extended clear CMOS. Unplug the PSU from the wall or board (Or switch it off in the rear), remove the CMOS battery, and then press and hold the case power on button for one minute. Then place a jumper on the Clear CMOS pins, and then wait 8-12+ hours. Then remove the short, put the battery back and connect or turn on the PSU and try again.
If you cannot get it you may need to RMA. Be VERY careful when shorting pins, you can blow out traces on the motherboard and that may void your warranty! I mention this because you mentioned shorting 6+8, that should not be done, and be very careful of that #8 pin when shorting as it is the ground pin (4+8 will blow a trace instantly, I just did that a few weeks ago accidentally!)