Your problem could be hardware related (HDMI cable or a finicky HDMI port on the receiver), software/driver related (I've read of people having problems using the HD4000, but not this particular issue), settings related (on your receiver or in Windows), or could be an HDMI handshake issue.
The only way to resolve it is to identify every possible source of failure that you can and then, one-by-one, try to eliminate them.
Things I would initially try:
1. Use another HDMI cable if you have one available.
2. Try another HDMI input on the receiver.
3. Doublecheck any HDMI-related settings in the receiver.
4. For the HTPC try a dedicated video card with HDMI if you have one available.
5. Try it without multidisplay, which I'm not really clear on. Do you mean you have multiple displays connected? If so, only have the amp connected to the HDMI on the video card with no other monitors connected to the card.
Don't judge a man until you walk a mile in his shoes. Then you'll be a mile away and you have his shoes too.