Dual Head RADEON MergedFB, overlay causes shimmer on screen0

[Bob <spam@homeurl.co.uk>]

When I playback video with mplayer (vo=xv), visit a site with animated 
flash advertising or watch a clip on youtube, my primary screen, (right 
hand side analogue over DVI, CRT 1600x1200@85Hz) shimmers, my secondary 
screen, (left hand side, analogue over VGA D-sud 15, LCD 1280x1024@60Hz) 
stays steady as a rock.

This still happens if I drop the resolution and frequency of my CRT to 
1280x1024@60 but it clears up if I disable MergedFB and use xinerama.

I have a Radeon 9250 (RV280) and I'm using the Open source RADEON driver 
(xorg.conf has Driver "ati" specified but that loads RADION) and I'm 
running Etch with Xfce with everything stock from apt (except flash, eww 
I feel dirty now).

Is this a known problem? Is there a fix? Google says not but my search 
skills may be poor today.

Thanks for your time.

--
Garrr, do your bit for global warming, become a pirate, you can "borrow" 
my copy of Windows 95 if you want.