Try alt+tabing to unfreeze it. Works so far for me.

Has it been doing this since the beginning when you first started playing the game?

There is a file that gets corrupted easily on this game, and whenever the game has any sort of weird glitch, or won't start, or anything else, the first thing I always try is to delete the local_settings.txt file. I am using Windows 10, but it should be located somewhere similar to:

This PC\Documents\my games\runic games\torchlight 2

It's safe to delete this file, since the game will make a new one as soon as you start it up again. You'll just have to reset whatever preferences you had for the game. Also consider that perhaps one of your preferred settings might be causing the issue, so you might want to play around with those a bit.

Another thing I've been doing with a lot of my older games is to limit the fps to 30. Doing this fixes a lot of "bugs" for these older games, since 30 fps was the norm during the years that the games were created. I use the Nvidia control panel to set the fps limit.