I'm sorry but I have absolutely no idea how to reproduce it, cause it seems to be completely random. The last crash was maybe an hour ago. And why do the Nervas's thrust change differently for each engine? That makes it impossible for me to use more than one in a single ship. (something with Actinides) I have to say that I'm a total noob in KSP and Interstellar.
and sorry I forgot.. I'm using version 1.8.14