This is a very common problem in WMC and I have tried to fix this for over 3 years. I have even tried selling old computer parts and switching out stuff to find the issue. Basically it comes down to driver issues, but you will never get it work 100% unless you install Gefen HDMI Detective Plus.
To start, I have Windows 8 with WMC running on HD4600 Graphics card with Onkyo Receiver in between HTPC and Panasonic TV. This has gotten rid of the sound dropping issues, sleep issues, No video card installed, HTCP issues, basically the only thing my wife complained about from switching to HTPC over cable box. This is a must have if your wife complains about stability because we haven't had to restart computer once in over a month since installing this device.
THE ONLY DOWNSIDE IS THE COST AT $100 BUT WILL BE MORE THAN WORTH IT
The awesome thing is that it literally takes a couple minutes to install and then you never have to worry about it again.
The one issue I speak of is the intermittent auto mute upon resume. Go here and I explain how to fix that if you run into this problem.
My Amazon review below
For all those who fought WMC and microsoft to fix the HDCP issues with drivers and updates, this will make all your issues go away!!!!!!!!!!!!!
So easy to setup - takes 5 minutes
No idea how this costs $100 (there is cheaper alternative on amazon you could try but this is the most common one)
Just go ahead and pay the $100, you will more than make up the difference by making your wife happy that she doesnt have to restart computer everytime she wants to watch live tv
ONE ISSUE -
It seems to be hit or miss on whether my computer starts up muted. Never had this issue before installing device but I dont care as its a simple button to hit on remote.