Sorry; my previous report said it's the gravity/traction that's weird.
I reckon it's more or different. The craft just wants to rotate all the time, it does very quick sommersaults and pirouettes.
Now, since it happened again, I'd say it's reliably happening for me on quick-loads. This was a repeat of the previous incident after calling a 'restart' on that one.
Sadly at this point I can't complete this mission if it keeps failing on a quick-load. :(