Daniel, Your experiences with the zippering noise sounds familiar. It might be that I got rid of the noise by changing cables or other gear. You jogged a memory: I know early on I had the midi i/o connected to the PC with a joystick/midi interface cable. At the time I probably would have used the on-board sound chip and maybe a PCI audio card (with joystick port). Later on I used a parallel port midi interface (MOTU midi express). Since that was decades ago, I can't say what I was using and what cables i/o ports I changed that helped with the zippering effect. Thanks for jogging the memory; I include it here in case it helps someone else.
BTW, the issue with the CZ-1 was brought up before, although there is no report of a solution here: https://groups.yahoo.com/neo/groups/CZsynth/conversations/topics/1648
Steve