There are a lot of (normally good) advice coming here, but the reality is that this is a software bug in Pillars that need to be addressed. Several of us have the same problem, at different places in the game. I can (still 2 patches later) play all of the game, BUT as soon as I enter level 9 of Od Nua, the computer restarts. Heat is below 50 on all meassurements at the time of restart.
When I can play all of the game (PoE that is) in sessions of more than 8 hours, leave the computer running Crysis demo or stress tests for both CPU and GPU for more than 24 hours with no problems, there is nothing wrong with the hardware. There is a (elusive) bug in there and it does things no software should be able to do.
It is nice helping each one encountering the symptoms with basic hardware error testing to eliminate any hardware problems, but I really would have expected some proper response to what is obvisously a problem with the game. If nothing else we should be told that since we're few experiencing the bug, and it (probably) is very hard to reproduce and identify (as the situation of the restarts are different) it is low priority.
I feel quite let down that I only get to play 2/3 of the super dungeon I helped finance during the kickstarter, and even more let down bu the fact that the bug seems to be ignored.