Z axis trouble

[JAZZCNC]

Test the voltage on the Direction pin while Jogging up/down. It should read 0v or close to it in one direction and around 5v in other direction.

If you get around 3V in one direction then it's probably your parallel port is low power 3.3v type.?
Even thou your old system probably worked ok the BOB could have had pull up circuitry to bring the pins upto 5V.?
If there's not a clear enough difference in voltage between high & Low the drives gets mixed signals and doesn't know which way to jump.? But it has to jump because there's another signal coming down the track so it just takes it's best guess and thats why you get erratic behaviour.!

If your still using the same BOB and your sure the wire connections are all sound and swapping to another drive cures the problem for a prolonged period then I'd say you have a intermittent faulty drive.?

One other thing to check is the PSU. If it's rated close to the minimum the Sys4 needs then it could be dipping below and some of the circuitry like Opto's etc are not getting what they need.? . . . . Maybe Clutching straws here and I'm no Electronics Geek but I've seen similar things before.!