Its very similar with my case. I did a lot research on the net about this problem, and one of sites was the gigabyte discussion of your motherboard model GA-Z77-D3H freezes suddenly every day .
Following the Intel_karla hint, i changed the voltage of my memory to 1.30v (1.25v originally) and no freezing all day long.
If you memory sticks is low voltage like mine is, try to change the voltage.
my thanks for your most helpful commenting, stability much improved now bit early to say gone yet but i have good feeling.
You are welcome, 2 days since i changed the voltage of memory sticks, and no freezing to report .