Can't keep SPS alive

[Jason McK]

I am wondering if anyone can help. I have attempted to keep Montipora digitata, Montipora capricornis and a Staghorn Acropora, I have yet to see any polyps from my Digitata I've had it about 4 months, with zero growth. My Capricrnis turned white and coraline algae grew over it with in two months.
I have a 75G with 2X250w 6500K MH and 2X 110 VHO actinics. My water parameters are good with zero ammonia and nitrite, nitrate is <5 and Phosphates are <.5
Ca is at 350 and Alk fluctuates from 7 to 10dKH and during the day my PH is steady at 8.3


Any thoughts would be great

Thanks
J