Hey, I had the same problem. Here's a solution that worked for me.
Before you load steam through PlayonMac, click the configure button on the top (it looks like a gear). There is an option in there to change the version of "Wine". I believe the default is 1.5.25 ( I could be wrong, regardless you need to change to different version). You need to change it to 1.5.23
This should allow the game to run without crashing. I did sometimes have issues running steam with this version of wine.
what I would do to fix this is start up PlayonMac with the original wine version (the one set by default, I believe 1.5.25, again could be wrong) and with steam open, change the wine version to 1.5.23.
Give it a minute or two to make the change then start up torchlight. This seemed to work for me when I was face with similar issues. My characters would save and my graphics looked great. Good Luck!