Based on what you said, it sounds like something on your PC is blocking the autosave function from working and causing a soft crash. Most likely an anti-virus software. There are a few things you can try as a work around.
1. Try moving the game out of your downloads folder and on to your desktop.
2. In the Mac version of the game folder there is a .swf version of the game this can be played with the Newgrounds player found https://www.newgrounds.com/flash/player.