I removed Inkscape from my PC some time ago, so I can only answer based on aspects I recall.
What you are describing sounds like and might be a RAM-issue. Sometimes there is enough RAM available to save the file, sometimes not.
So there might be situations where your 8GB RAM is not sufficcient. You can try to free RAM by closing any programs not needed when working with Inkscape.
Are you on a 64bit Windows and do you have the latest 64bit Inkscape version (0.92.2) installed?
For dealing with large files the 64bit version is of course recommended.