Thanks for all the replies, but I have finally solved it. After using many different clock speeds, voltage settings, etc, I finally discovered the memory timing levels.
Wow, what a difference! Now the same card runs at 30mh/s and at a constant 75 degrees (its hot summer in Hong Kong).
Now this rig, 7 x rx580, runs at 207mh/s using 580W power.All temparetures around 75. Nice and stable.

core clock: 1150
core voltage: 850
mem clock: 2125
mem voltage: 900
mem timing level 2

All these settings are now done in Radeon software, rather than Phoenix. I seem to get better results this way.