Multigrid convergence

Hi all;
I am solving a 3D Poisson Equation by linear multigrid. It works fine when dx=dy=dz, but when the sizes of dx, dy or dz are not equal (the ratios are not unit), I get instability problems. The Poisson solver convergces slowly ,or sometimes doesn't converge. I have the same problem in 2D and also with FAS-Multigrid. Is it an usual case in Multigrid? Is there any factor to adjast to have better results from non-square cells?
I will be thankful if somebody can give me a hint.

Michel

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