to recap, is this correct?
* System boots to Windows with no card, iGPU only.
* System POSTS but hangs with dGPU slotted but not powered auxiliarily (PSU-direct)?
* System does not POST with dGPU powered properly by slot and PSU-direct? or is it booting but artifacting in this config?
I would replace the Rosewill PSU tomorrow, no hesitation, even with a cheap EVGA. It's certainly possible the video card's power delivery circuitry has aged or failed. Sounds like its time for a Monoprice cable or adapter shopping spree. $10 should do it. Might be worth it.
1. Correct, no discreet graphics, and with the IDP disabled, using the IGP (vga) = boot into windows, no issues
2. Correct, without power it'll post but then blackscreen prior to even a windows 8 logo or loading screen, and stay there. With power (2x 6pin molex) it'll reach a windows 8 logo, then a "diagnostic screen" to repair, which fails, then presents the "restart, repair, refresh, etc" menu.
3. Incorrect, it POSTs if powered properly, but displays green artifacts directly after POST and prior to windows logo - then behaves similar to the latter half of 2. Only reaching the windows login once, and I don't know why, but regardless artifacting persists on any screen that does display. I also don't understand what it's trying to repair or diagnose every time the card is powered.
Without the card plugged in, I installed the latest GPU drivers for the dGPU which did nothing once plugged back in. I also attempted to grab the latest chipset drivers, but apparently Gigabyte's chipset drivers for this are rather poor and only want to install the Catalyst Control Center and some display-related drivers, which I don't want.
I took the entire GPU apart, removed the heatsink etc, no damage to any ram chips, no scorch marks anywhere I could see, no blown caps, no bulging caps, no bad traces as far as I can see.
I'm going to take my PSU from my system and try it, that's probably the easiest test to rule out bad power. I'll assume the slot itself is OK because it must be receiving some power from the slot to allow it not to boot when the 2x molex are not connected.