Thanks John, I could not find a way to update the BIOS of my old card so today in desperation purchased an up to date model which presumably has a reasonably recent BIOS. Alas, it behaves exactly the same as the old one.
I have wasted so much time on this display now I am getting frustrated in the extreme.
After decades of trouble shooting electronic gear I've grown highly suspicious of any components I cannot verify as compatible. And I've always learned to always put a meter across visible fuses. My last sentence is more of an inside joke but it says it all. You probably remember glass automotive fuses of the past, with the filaments we could look through and visually see? We presumed that if we could visually see an intact element then the fuse was good. But a new technician (at least the ones who went on to become good technicians) quickly learned to put a multi-meter across the fuse to check for continuity DESPITE our presumption. We'd quickly learned, sometimes after spending unnecessary hours, days or even weeks on a problem not to mention untold expenses from ordering other stuff in our quest fro a repair..that the only SURE way to check a fuse, is to put a meter across it. This mindset taught us that we'd ALWAYS spend less time trouble shooting overall, and that just like Mr. Murphy lurking in the background waiting to make his presence known, that the first time, even in years, that we failed our due diligence with the meter.. it would bite us in the ass.. Usually to the amusement of our co-workers or the ire of our supervisors.
So, while I agree what I'm going to say on the surface seems unlikely at best.. it's the way I've learned to trouble-shoot after decades of experience, and because I do I'll never spend an unnecessary hour OR dollar chasing a bad fuse.
Based on everything you've said in this thread so far I would:
1. Have you physically changed out the DVI cable?
2. Power: You've inspected or verified your power cable and you're NOT using an extension?
3. Your power supply in the computer itself at 600 watts is light for your build. Have you at least checked it for proper voltages under load and ripple? Ideally you should be 80% of your load at 50% of your rated power assuming average usage patterns. More and you generate excess heat, excess fan noise, and stressing the supply more less than ideal outputs. I do this a lot so I keep a few known good reference supplies so if I'm in doubt I can easily substitute, if you don't do this a lot a decent meter isn't terribly expensive and can be used for many things.
4. CALL your video card manufacturer and ensure you're working with a compatible firmware. UEFI BIOS sets are still new enough where a newly purchased video card might not be compatible, especially if you shopped by price.
Or as a poster suggested, just replace it. I try to discourage this, even if it's what I'd do myself, because it's more economically and environmentally responsible to replace LESS gear resulting in LESS full landfills.
Plus, sometimes the thrill of the hunt can be well worth the extra efforts..
I understand your frustration and would support you if you wanted to use your discarded Apple product for target practice.