I've had the XFX R9 390 working for about 6 weeks in a Windows 7 machine with two monitors, both using DVI-D. Worked great.
Now, only one monitor will display. One is set to 1900x1200 and the other 2560x1440. If the the larger monitor is on, the card shuts the signal off to the smaller monitor. If I look at the screen resolution control panel, I can see that the card is constantly attempting to detect the other monitor.
If I turn off the larger monitor, the smaller monitor gets a signal and displays at the correct resolution without a problem.
I've switched cables, switched the display outputs, booted into safe mode and uninstalled the driver software. Re-installed the driver software. No joy. At boot, they both display the boot messages fine.