I had my Rpi up and running using Raspbian, connected via HDMI to the TV and running at 1920x1080p.
Everything was working fine, then I installed Raspbmc on a separate SD card and switched to using that. It installed and started up fine, but whenever I use the keyboard to navigate the UI, the screen goes blank for 1-2 seconds before reappearing. If I try to use the mouse, the screen goes blank every time the mouse moves. It's as if redrawing the screen is taking seconds rather than milliseconds.
Worse, if I navigate to the Appearance screen, the screen goes blank and doesn't recover until I hit Esc to navigate away from that screen.
I've edited the config.txt file to reduce the startup resolution, first to 1280x720, then to 640x480, but it hasn't fixed the issue. I've searched these forums, and the Raspbmc forums but haven't seen this issue reported.
Is there something else I can try? Maybe installing a more lightweight skin via SSH rather than via the GUI?