You can ensure the GPU card VBIOS isn't buggy by reflashing it. It's an elderly card, perhaps the firmware is corrupted, certainly the warranty is long expired, it already doesn't work properly, you really don't have much to lose with the effort. Not much you can do if the VBIOS firmware actually contains buggy badly-written code, but this would correct VBIOS firmware which has somehow degraded or been compromised from a working state to a nonworking state.
As Qwinn stated above, a cold reboot with all power fully off (or even disconnected) can sometime remedy borderline firmware devices. Old bootloader components can be finicky and cantankerous, sometimes they need a hard kick to get going.
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