I can run the machine for hours without any issues. I've got max velocity for z axis set to 30ipm. During tuning,it will not slip. In fact I can increase the speed to 60 without slipping under heavy pulling.
Yet when I use the feed override, the z axis will sooner or later go crazy. Before I can catch it, I find it digging into my sacrificial board.
It's random, so I cannot duplicate it. When I am doing a nested job  that lasts appx one hour, it will inevitably do it.
Once I restart the machine, recalibrate it and keep it without any feed override, I can run the machine successfully.
I am puzzled, since there are times when I want to go faster without having to hard code the feed speeds. It never happens with the x or y axis, which run at 99 ipm.
Any ideas? This is on Mach3.
tony

There is a known issue with Feedrate Override, if you use it to increase feedrates by a large amount.
It's best to program higher feedrates, and turn it down to start with. I would not try to use it above 150%.

That may explain the issue. Thanks.
tony