Z Axis problem with Marlin Firmware

[CJiwan]

I'm using a Reprap printer with Marlin Firmware.
When print a part with Simplify3D, the 2 first layer are printed perfectely. A third layer Z axis down instead of up.
This also happens if I slice with Simplify3D and print from the Repetier-Server the generated Gcode.
If I slice with Slic3D it works perfectly.
I think the error is created by simplify3D in the GCode/

Thank for your help

Christian

[CompoundCarl]

Well, there's a pretty easy way to troubleshoot this. Just export the gcode file and then open it in a text editor. It's pretty easy to search for any "G1 Z" commands, so you can see exactly what the software has written. That way you will know if it's a software or firmware problem.

Personally, my guess is that your firmware's Z-axis inactivity timeout is too low, so the axis is dropping while you're still printing