CNC router occasionally makes jarring noise and home jumps when it does this. Help!!

[itsmillertime]

Hi Guys I was hoping that you could help me track down the problem behind why my CNC 6040 router now and then (but pretty much most jobs) it will happily be machining away. When at some point the machine will make a louder 'jarring' kind of noise and it appears to be along the Y axis by what it does to my work pieces (the longest axis which the gantry moves across). The machine doesn't appear to stop but it looks as though it jumps by about 10mm. I haven't been able to physically see this by eye as everything is always too quick and it's not spectacular enough. But I can see it jumps on my work.

You can see from the photos attached what it is doing.

The machine appears to be operating just fine until this happens i.e circles are all correct as are square shapes.

I know that they have lots of quality issues, but I would be really grateful to you guys for advice on how to diagnose this.

Upon starting up the CNC router and running mach3 on my desktop all 3 stepper motors lock as they should and the router happily runs/jogs along all axis. I can turn the stepper knobs by hand freely and cannot feel any rough parts of travel in the ballthreads.

Please help me diagnose this with simple steps. Is it more likely that it's wiring or a stepper motor or driver?

Just guessing possible Y axis wiring issue. But would love to see what you all think.

As you can see this error has made it into all 3 jobs below. Looking forward to hearing what you guys think.

Cheers andy