I had the same problem when I capture from Xbox 360 with HDMI in 720 or 1080p (bitrate 15000 Kbps). With the PS3 in 1080i (video component cable) it does not happen.
My Desktop PC is a Intel Core 2 Duo e7500 @ 2.93GHZ with 4GB RAM and Windows XP
On my notebook that has processor Intel Core i5, 4GB of RAM, Intel HD Graphics 3000 and Windows 7 and the same thing happens!
Sometimes the problem happens more often, sometimes less, but usually in moments of intense action gaming.
I've tried turning off the anti-virus, the program Booster Razor, but none. As I said, only occurs with the Xbox 360 HDMI (720 or 1080p).
I ask for urgent action from the part of Roxio towards solving the problem (maybe some software update)?