I have a similar issue to you, I'm not entirely sure what the problem is, but I figured it might be heat issues like the others have said. So I've made sure my PC is well ventilated, I removed the front cover from the case, I've put on a customer fan curve for my graphics card, I've sped up the case fans and touch wood I haven't had an freezes or crashes yet. Though I'm still not sure exactly it is a heat issue, I think it could an issue with a USB device(Wacom Tablet), or the PSU. Stuff like this is really hard to diagnose, best of luck to you.