Hi @tech3ric. You mention that P can boot (but not Q), but that you tried to flash the stock bios onto P(?). Do you mean you tried to flash the stock bios back on to Q?
By no means am I a pro here and I recommend researching as much as you can prior, I’d highly recommend:
A) Boot using P. Once booted, flip the switch to Q (the bios I’m assuming you attempted to flash with the mod).
B) Follow the flashing steps outlined above, making sure that you’re just trying to flash your stock (e.g., backed up) bios back on to Q – no mods, just try to get the card working again.
C) If you still get a black screen on Q, repeat step A then see if Son of A Tech’s video helps you out (the pertinent stuff with the Sub ID in MPT starts around 6m:43s). You may need to copy the Sub ID from your stock bios (or the stock bios that’s remained untouched on “P”) into the bios you’re attempting to flash.
Best of luck! Remember to try and keep “P” as pristine as possible. Double check the switch before you attempt any more flashes.