So you think it is the rock because you eliminated the other possibilities? Do you also have substrate in the tank?

Perhaps if you take a piece of rock out and put it in another tank that you know has zero phosphates, and then test that water after a week of just having a piece of that live rock you could verify it is the rock.

Are you also sure that you are using enough of the GFO and replacing it frequently enough?

Also what test kit are you using for your phosphates? I assume you are having a big algae issue?