This is normal and despite the experience of some to whom it nevers happens (and I believe this is so), it is a common occurence for many others. Yes, it is a bother to have it close unexpectedly, but it truly does not harm your file.
There are signs you can look for to prepare for the closing. For example, if media file or some windows are outlined in red that's a sign it will close soon. If some of the media previews are blank that's also a sign but not always. You can also keep an eye on memory usage using Task Manager. You'll soon come to know how much memory is too much. Close some other programs like Office programs and Internet Explorer that can also use large quantities of available RAM. Be proactive. When you suspect the program is about to close, close it yourself and start again.
Again FTM 2012 can be a pain to run, but it is really the best program for its purpose out there. I think if you just come to terms with the memory issue and manage it yourself you'll come to prefer FTM 2012.
By the way the difference between 32 and 64 bit is irrelevant. FTM is a 32 bit program so it can only use that memory. And 32 bit machines can only use up to 2G of RAM so it restricts the amount of RAM useable.