Thanks for you comment, as it made me realize I had not correctly
specified all my boundary conditions.
It should have been:
bc5:v(L/2) = 0;
bc6:z(L/2) = 0;
instead of:
bc5:v(L/2) = 0;
bc6:v(L/2) = 0;
The old bc6 was of course redundant, and therefore there were only 7
equations for 8 unknowns. I guess I was just too mystified by the label
to realize the typo.
Thanks,
Pete