I have always had a minor issue with video playback in Windows Media Center. It is a very minor issue, indeed, but still somewhat annoying. Video playback generally works perfectly, irrespective of the source (DVB-T, DVD, ...), but when the Windows Media Center window is not active, that is, when it does not have keyboard focus, then video playback is choppy.
My computer is very high-end (at least it was when it was new, a couple of years ago): Intel i7 CPU, 6 GB RAM, Windows 7 Home Premium (64-bit), 1.5 TB HDD, etc. I have had several graphics cards. The two latest are based on the nVidia GTS 450 and nVidia GT 640 GPUs, respectively. I have three displays connected to the graphics card: two regular flat panel screens (DVI) and one 46" wall-mounted LED TV (HDMI). The issue is the same on all screens.
Is this a known issue? Is there a solution (well, besides always having the Media Center window focused)? Nedless to say, with three screens it makes perfect sense to run Media Center full-screen on one of the monitors, and doing other work on the others.