(In reply to comment #8)
> Okay, but doesnt that mean in this case it is a problem with the display
> (HDMI?) clock, as I am not using HDMI audio?
> 
> Is there a way I could get more detailed logging of what is happening on my
> system?

I don't know how XBMC calculates the frame rate off hand.  The PLL used to
generate the display clock may not always match the exact pixel clock of the
monitor.  The driver calculates pll dividers to get as close as possible to the
pixel clock of the display mode.  See radeon_compute_pll_avivo() in
radeon_display.c