# Blowup of error in Conjugate Gradient method with periodic Dirichlet Poisson matrix

My problem is that the L2-Norm of the residual for the periodic Poisson matrix $$P$$ is initially decreasing but starts to blow up after a certain number of iterations. The blowup happens earlier the larger the matrix is.

On the other hand, the Poisson matrix $$H$$ with homogeneous Dirichlet boundary condition converges exponentially for many iterations without any notable oscillations.

For reference, I am using the CG algorithm presented here from Wikipedia.

The only criterion for convergence I came across seems to be the positive definiteness of the system matrix. What could possibly lead to the blowup with matrix $$P$$?