Nicolas,
I don't have a "specific" answer for why your machine all of sudden plunges down. It may happen for a number of reasons. I am not going to tell you to slow down / speed up/ retune ......... Instead just realize the following:
1.Not all material ( wood ) is the same so you need to adjust feeds and velocity based on what you are doing
and working with. No difference than if you were doing it by hand.
2. You can be missing a step. Normaly you would imediately hear it , but , because of the load router noise , you probably wouldn't hear the missed step.
3. My experience on missing steps is , two things happen, only a very small change in the axis location or it's total loss of position and away she goes with nothing to control it except the estop.
During cutting air you can usualy just change velocity or acel leaving some headroom for cutting and thus not have a problem. That may not be the case when cutting, as now you have introduced another parameter that can change the axis motion requirements as compared to air cutting / rapid move.
Nothing beats just cutting some scrap of different materials at different depths/ speeds / feeds to get a good feel of what to do in a program, AS IT RELATES TO YOUR MACHINE.
4.A router throws out all kinds of rf / noise / intreference / whatever you want to call it.......
You can be using it for a long time and then all of a sudden the noise will get ya ! Proper grounding, shielding , etc will go along way to minimizing the chance that it can happen.
5.Now you also have the machine, suppose you jog the Z up and down or whatever 1000 times and all is well.
Now you cut something and at some time crap gets in your ways, or screw, etc ....and all of a sudden missed steps
because of mchanical influence.
6. I don't think it's the software, but one or a combination of some of the above.
FWIW,
RICH