It is the handshake issue. PC does not see anything on the other end and shuts the connection down. This cause me untold aggravation with my HTPC - it would 'forget' about the AVR and TV any time AVR was turned off or switched to another input.
The only solution that REALLY worked was Gefen HDMI Detective Plus This (currently) $85 tiny box goes between PC and AVR. It captures EDID from AVR, and stores it. Then, it always presents PC with this EDID, regardless what state AVR is in. PC just thinks that AVR is always connected, powered on, and set to input from PC.