Blender crashes on old files after some time

Hi there
I’ve got a problem, that I could not find in this forum yet. 2 years ago (2.49b era) I started some files for a project, but I left them idle on my harddisk drive until needed, which is now.
It all seems to work fine. I can open them with any new version of blender just fine and do some editing, save them, reopen them, work some more, save some more.
Then at some point, after having saved the files say 5 times, blender will no longer open the files and crash instead on loading them.
I cannot make sense of this behaviour.
After that it is still possible to create a new blend-file, append the scene from the crashing file, save it, reopen, save, but then again, after some time working on the file, it will crash blender on startup.

Since it happens with all new versions I guess it might have something to do with that, but the strange thing remains that it is possible to work on the files for some time.
I haven’t tried working on them with an old version, especially since I waited for so long to be able to use new functionality like indirect lighting to achieve more natural results.

Has anyone of you encountered similar problems or knows of a debugging procedure?

I work on a Win 7 64bit AMD 6 Core and ATI graphics card. The old files were created on an Intel 32 Bit machine (in case that is of any help).

Your insight and or help would be mightily appreciated.

All the best,