Shanglong,
I can reproduce this behaviour, but this isn't a bug. I output the 162x162 
system matrix at first cycle and as far as I can see it fails symmetry 
check. You can see this already from first few non-zero entries:

(0,0) 1.33333
(1,1) 1.33333
(2,2) 1.33333
(3,3) 1.33333
(4,4) 1.33333
(5,5) 1.33333
(6,0) -0.666667

See, there is no (0, 6) and symmetry is violated.

The matrix is still positive-definite, that is probably why CG still gets 
something out of it. 

Alexander

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