Multigrid stability problem
 

Hi all; I am solving a 3D Poisson Equation by linear multigrid method. It works fine when dx=dy=dz. But when the sizes of dx, dy or dz are not equal, I get instability problems. The Poisson solver convergce slowly ,or sometimes direvges. I have the same problem in 2D and also with FAS-Multigrid. Is it an usual case in Multigrid? I will be very pleased is somebody help me to fix this problem. Thanks in advance.