what psu you using
Platimax 1500W EPM1500EGT
http://www.enermax.com/home.php?fn=eng/product_a1_1_2&lv0=1&lv1=52&no=191
now just one minion to that power supply
That's it, your PSU is very powerful but not single rail. Your PSU had six rails, each rail only can deliver 30A max.
Using default setting the board will use up to 40A(i guess since i don't own one
)To prove this you can try to disable 1 chip & see if the board can reach 300G
You can see more detail on cgminer api log.