Have you checked your geo? As Oshyan says, obj is currently the most reliable way to import geo from an external 3d app.
Often times the geo is "dirty" and should be cleaned before import, for instance when exporting geo as an obj from Terragen, each vertex is duplicated, and verts can be merged reducing file size drastically.
On the whole though, I'm very impressed with the way handles large amounts of geo, and provided it's clean, I've not had any issues in the past.
Also does you terrain have UV's? If you plan to use your imported geo with Terragen's procedural textures you will not need UV's and they can be deleted on the source object before import.
Checking for those issues could reduce file size significantly (I've seen reductions from files the same as yours - 1.6gb to about a fifth of the size)
Might help you with your import instability issues.
