Rare random freeze on a new PC

Got this PC a few weeks ago and happy with it apart from this rare issue. It happened for the second time today. The mouse cursor begins to move very slowly, like it sticks in one spot then jumps to a new spot rather than gliding across the screen, then the computer freezes entirely and I have to hold down the power button to force shut down. Both times it happened with nothing running except Firefox. First time I was watching a video on Youtube, just now it happened as I was reading on Wikipedia. The fans aren't going wild or anything when it happens, everything sounds normal, it just freezes. I don't even have STEAM or anything running because I have all those set to NOT start when the system starts up, it was literally just Firefox.