Check the Event Viewer to see if there are any administrative alerts -- that may help isolate a cause (assuming the system has time to write an event before it freezes).
On the hardware front, you could try reseating the memory and the video card (just take them out and put them back in, making sure they snap into place). Sometimes systems act weird if the components aren't completely in their sockets.
Main System: i7-8700K, ASUS ROG STRIX Z370-E, 16 GB DDR4 3200 RAM, MSI GTX 1080 TI, 1 TB CRUCIAL MX500, Corsair 550D
HTPC: I5-4460, ASUS H97M-E, 8 GB RAM, GTX 970, CRUCIAL 256GB MX100, SILVERSTONE GD09B