Are you shutting down all non-essential programs (especially anti-virus) before starting the game?
There was one conflict with Daemon Tools reported which cause frequent crashes (every couple minutes, in that case).
Have you tried lowering the graphics settings and resolution, etc, to see if that would have an impact?
One person reported stability problem if they had Ambient Occlusion enabled, but the game was fine if they disabled it and enabled it in their graphics drivers. You could try disabling all the post processing in the graphics options, etc.
Do you have Vsync enabled, or the frame rate cap set in the options? A couple people reported overheating on the main menu or character creation screen due to a very high frame rate, even if it was fine once in the game itself. If heat was an issue (doesn't sound like it), then you would probably be able to play longer when you first turn on your computer, and if you restart the game immediately after crashing it would exhibit problems sooner.
What refresh rate is your monitor running at? Someone reported crashes when casting spells if the picked a resolution at 59Hz refresh rate, but the game was fine with the same resolution at 60Hz.
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, any crash dumps, and your saved games. If file size is an issue, you can delete most of the saves from the zip.