Had the same problem but worse now. Card (Sapphire R9 290X Tri-X 8 GB, 2 years old) is still running in VGA-mode right now but can't install ANY driver since 4 days. It started with black screen on booting after the windows logo, before login-screen. Tried to install driver new then and reinstalled win 10 after the driver installation didnt work (any driver). Found a lot of treads about this issue in this forum and in others - most don't lead any further. Wrote to Sapphire and got a short answer: Try card in different PC or card is damaged. Lot of people say its the VRAM that is damaged. Some say "baking" of the card might help to fix corroded soldering. It really helped some people I saw in YouTube and also read in this forum. I didnt try it. I bought a new NVIDIA card now with 5 years warranty from Zotac.
I think your card might be on the way there (hope not). If you still have warranty return it.