I've been looking into this issue for my GX502GW which exhibits a very similar problem to OP. I think this problem might be affecting numerous other laptops in the ASUS lineup with Nvidia Optimus. FYI - Before the below testing, my graphics mode is "...

Posting this in a few threads to spread awareness: I've found that the Windows Update catalog retains historic BIOS information along with other driver details for ours and many other machines. Somewhat expected but lesser known information I suppose...

Posting this in a few threads to spread awareness: I've found that the Windows Update catalog retains historic BIOS information along with other driver details for ours and many other machines. Somewhat expected but lesser known information I suppose...

Once again I return with more information. I've found that the Windows Update catalog retains historic BIOS information along with other driver details for ours and many other machines. Somewhat expected but lesser known information I suppose. Anyway...

Sorry me again, I found that Windows stored a copy of the 306 BIOS in C:\Windows\Firmware This is for the GX502GW: https://1drv.ms/u/s!Aq-uXsl5ag1swAjkY7P6fcOVVwUB?e=1ulILl Just used it today to downgrade.