stick with the 680i until something better than the 790i/x48 comes out.
you should easily get the qx6700 to 3.4ghz on air. ive mine on 3.6 on watercooling.
im also using a striker 680i and i havnt turned my machine off in 4 weeks. also using 2 150gb raptor's. huge transfers from utorrent and across my network.
pc hasnt missed a heartbeat so id say its a fault with something. possibly windows if its dying after 48 hours.