Vectorising nested for loops

I need to create slices of 2d matrices(ie a 3D matrix Y in which each slice is a 2D matrix )from a given 2D square matrix (which is the matrix X)..Dimensions of X can be upto 512.If dimensions of X is NxN, then dimensions of Y is NxNx(N/2).

The original code is as follows

    N=size(X,1);
    M=N/2;
    Y(1:N,1:N,1:M)=0;
      for k1 = 1:N
        for k2 = 1:N
          for p= 1:M
            for n1=1:N
              for n2=1:N
                N1=n1-1; N2=n2-1; P=p-1; K1=k1-1; K2=k2-1;
                z=mod((N1*K1+N2*K2),N); 
                if (z==P)                                       -->A
                Y(k1,k2,p)= Y(k1,k2,p)+ X(n1,n2);
                elsif (z==(P+M))
                Y(k1,k2,p)= Y(k1,k2,p)- X(n1,n2);
                end
               end
             end
           end
        end

Execution time for a 64x64 input is 28.9 seconds. I managed to vectorise the inner two loops and the if statement

The modified code is

     N=size(X,1);
     M=N/2;
     Y(1:N,1:N,1:M)=0;
      w=repmat((0:N-1),N,1);
      q=w'
      for k1 = 1:N
        for k2 = 1:N
          for p= 1:M
           P=p-1; 
           K1=k1-1; 
           K2=k2-1;
           z = mod((K1*q)+(K2*w),N);
           Y(k1,k2,p) =sum(X(z==P))-sum(X(z==P+M));      
        end
      end
    end

Now the execution time for a 64x64 matrix input is reduced to 20.2 seconds.

I need to vectorise the remaining 3 loops also..Please help...I need to bring down the execution time to minimum..Thanks again..