With the X11/X13 algo, I noticed my R7 260X is stuck at 150Mhz memory clock speed without any monitors being plugged in. If I plug in 2 monitors to both ports then it runs perfectly.
It doesn't do this on Scrypt or Scrypt-N
When I unplug the monitors, it downclocks the memory clock. It does this in Windows and Linux
im not wuite sure why this is happening to you , but the easyest fix would be 2 "dummyplugs" to simulate connected monitors.
you can browse instructions easily here on forum.
i have some 270x, but dont have any problems what so ever, are you sure you are on latest catalyst ?
cheers