I've been having this problem since last year and I've just been working around it. Its essentially a single prefab that is crashing Radiant at random when I map in it. It could be from selecting a single brush, to lasting 10 minutes and then crashing after tons of brushes are selected. I have tried searching the file for "QNAN" and got nothing
 
I can almost always get it to crash when I just spam select brushes, I'm thinking it could be a broken brush or something that is just bugged in the prefab? Any ideas would be greatly appreciated as always Thanks.

How many mb is the .map?
any errors at all when compiling?
any other errors in radiant console when loading the map?
Is it freezing when it autosaves?