Problems with 2D poisson solver and periodic conditions

Hello, 

I am coding a fluid solver in the Vorticity-Potential formulation. I need to solve a poisson equation for the stream function. I was trying to use MKL's. In my problem, I have Dirichlet BCs in the y walls and periodic in the x walls. When I solve the poisson equation with the MKL solver I get wrong results at the corners, but when I solve it using SOR I get the correct results.

 I know that I am using the solver appropiately because I have tested it with other functions and I have gotten correct results.

I can see  from the results that the periodicity is not being enforced when MKL is used. 

I am using Intel Visual Fortran XE 2011 Update 10. I have attached the code, I use the variable useMKL to switch between MKL solver and SOR. It would be great if I can use MKL's as I need to move to a 3D implementation and the SOR method will be very slow then. 

If more information is needed let me know. Thanks in advance,

Camilo