DMG files?

Lately, when downloading files, it has been a guessing game as to whether or not I succeed. Sometimes I get the file and either Stuffit Expander or the Unarchiver will open it automatically or it comes in as a .dmg/binary file that cannot mount or be opened by said programs. I would assume in normal cases that this is just because of the file, and I can't download it, but a lot of times I'll try again at a different time - and not always on a different connection - and it will work. Whats wrong?

This might be a browser specific problem, turn off post-actions on downloaded files.

If you use safari:-

Go to preferences and untick open safe files.