Was this with the free Tech Preview 2, as with your previous renders? We will release an update to the free version soon, and a lot of changes have been made which may help.
Could it be that you're adding more to the scene than you did in your earlier renders? Another possible factor is using Tech Preview for a while and then starting a render without closing and restarting TG first. (You shouldn't have to restart it before a render, but often it does help). If the scene is particularly problematic, sometimes restarting the computer before rendering can help too, to avoid memory fragmentation by Terragen or by other programs.
Matt