Random hot end plunge!

[hmanvel]

I'm beginning to think my system is possessed. I'm having random build failures, and I've never been watching when they occur, but today I have evidence. It always goes like this; it will be building fine, until about 1/4 - 1/2" of layers are done, then the hot end will suddenly plunge down, trying to get to the bed (or below? don't know) then when it jams there, it pops up and starts printing in the air. I then have to re-zero the z height, and sometimes have to do that several times to get things working again. I'm attaching a picture of today's fiasco, you can see where the divit and burn marks are, where the plunge occurred. What in the world should I look for to cure this? This is a Rostock Max V2, with the E3D V6 hot end.

[Mac The Knife]

That sounds like a combination of matercontrol and Slic3r.