I have to go out on a job now, but I'll try that later. It's been homing fine, including a rapid move to 0,0,0 and it's repeated fine. I think this is to do with the setup of the Z move when it's on its own. If you halve the Zmotor tuning acceleration the fault remains, but if you change the maximum Z velocity to 1000 and leave the accel at 1000 then the problem goes away.