The ZX/MZ series is plagued by having a plastic gear on the mirror motor which splits and slips. This is typically manifested by the mirror locking in the up position. The operation of the aperture, shutter and film advance are all inter-dependent on the timing of the mirror operation so it is not inconceivable that a partially slipping gear would cause the problems you are seeing.
If the gear is slipping the solution is to replace the gear with a metal one. See more details in this thread:
MZ5n mirror blocked, service manual? - PentaxForums.com