Hmm...Have your scenery projects been placed in your Addon Scenery folder, and activated in FS accordingly? How large are the new scenery files? The city project I worked on ended up with about 5 MB for all of the .bgl files, with another 35 MB for the associated textures. Even this amount doesn't cause problems such as a memory crash. Also, your amount of RAM seems more than up to the task. Unless you have been
very busy with SBuilder lately

then maybe check out some other things, such as looking up which processes are running in the background, and whether the problem persists if your addon projects are temporarily removed. Hope you get it sorted out soon.