4 input CMOS NOR gate simulation showing metastability
The P-tree (pull-up network) needs to be driven the same as the N-tree (pull-down network). The circuit is a four input NOR, but you are driving it with 8 inputs. The result is that sometimes neither the P-tree nor N-tree are conducting and therefore the output is floating. Or, both the P-tree and the N-tree are conducting causing the output to be mid-level (and lots of current consumption). 

The proper way to test a 4 input NOR is to use only 4 voltage sources and drive I0=I2, I1=I3, I4=I6, and I5=I7.