My was actually the opposite… 
Combine mesh worked perfect, when I separated the mesh’s (attempting to get high-res lightmaps on each object) is when it started to run out of RAM & crash.
It’s still very much a learning curve for me. My previous tests were of much smaller spaces so attempt is to now scale it up & see what issues I face with regard to quality…
FBX here: http://www.filedropper.com/house_3 - I split up the walls, ceilings and floors and set the lightmap res to 1024 which is when it was failing.