G.H.
A while back I posted this :
http://archicad-talk.graphisoft.com/viewtopic.php?t=2744
after experiencing nondesirable results. While not exactly the same as yours I suspect they could, and are likely related. Note I am still using 8.1 but I believe the sketch engine other than rendering speed is the same basic product. Hope that helps or at least leads to better understanding of how to drive this vaulable tool within ArchiCAD. Actually I tend to think of it like an old truck, you guide it more than drive it. Please do share any further insights you come up with.
jeff white
w3d design
AC 23 Solo US / current build & library
Windoze 10 Pro 64
HP ZBook 17 G4
Intel Zeon 3.0
Twin 2GB SSD
32 GB memory
http://w3d-design.com