I switched the monitor that wasn't having problems to DisplayPort and that solved the issue.
It doesn't really answer the question about why the card was working fine for 5 weeks and then suddenly wasn't.
It feels like the DVI port went bad.
This may have solved my problem, but doesn't answer WHY my card worked fine with both monitors connected via DVI and then suddenly stopped.
Is it a bad card, or a bad driver?
I'd prefer an answer because, a bad card tells me that eventually this solution will stop working as well.
An update: (in case anyone can help)
I had 2 monitors connected to Windows 7 and an XFX R9 390 with DVI-D cables. After a few weeks, one of the monitors stopped working. After swapping cables, testing the monitor, both monitors and cables worked individually with the card, I switched one monitor to Display Port and suddenly both monitors worked again. For a few weeks. And then the second monitor, still using DVI-D, stopped working. No amount of port switching, cable switching etc could get both monitors to work at the same time. Revert back to 6970 card, everything is great.
So, fresh reinstall Windows 7. New 1000w power supply (only thing in this box is 3 sata HDs, a dvd drive and video card).
Start with the R9 390, install latest AMD drivers, works great. For 3 weeks. Second monitor drops out. This time, there is no way to get both monitors to work together with this card. Try another new power supply. No change.
Thoughts? Why would it work fine for a few weeks, then stop?