Problems with viewing screens in multiple monitor configurations

Question

Sorry if this is already posted somewhere, but after searching for 45 minutes, I couldn't find an answer.
Since upgrading an iMac to 10.5.7 I've had problems viewing over remote desktop. The iMac has a second display (20 inch Cinema Display) and mirroring is off. Before the upgrade, selecting which monitor to view was a snap. After upgrading, if I select one monitor after about 3 seconds is switches to a 2 monitor view. It doesn't matter which monitor I choose. When it displays both monitors simultaneously, the extra monitor's screen will flash every 3-5 seconds (via ARD, not in reality) and the mouse connection will freeze for about a half second.
Obviously, this makes controlling the machine difficult.
I've trashed all screen sharing and ARD related preferences on the machine without solving the problem.
If I turn off remote administration in the system settings and activate screen sharing, everything works fine (but I lose the other important features of ARD). No other computers on the network have a similar problem - but this is the only machine with 2 monitors. If I turn on mirroring, I still have the problem of the mouse control freezing every few seconds. I assume the connection is being dropped and refreshed.
Any help would be appreciated.
-Ben

Answer 1

Same thing here.
From the instructor workstation I can see any of the lab computers when viewing both of their monitors. When I try to look at just one monitor all I see is a black screen.
The video mode on the instructor workstation seems to be important. The instructor station has a second monitor that is a 1024x768 video projector. We like to have video mirroring on, of course. But in this mode, the student's screen always displays black.
If I switch mirroring off, I get the same thing IF I pull up the student's screen on the projector display. But I can see it if I pull it up on the local display. And it still shows when I move it over to the second display. However, if it's on the projector display and I try to select a different display it goes black.
But the whole thing is kinda flakey. Sometimes it works one way, sometimes it doesn't.
It works better if I set the resolution on the external display to 1600x1024. But that looks badly stretched on the screen.
All of this started happening with the latest ARD update.
I tried the same thing from a MacBook and it works great.
I suspect that it is a mismatch between ARD and the video card in the iMac. (Model Identifier: iMac7,1). The Video card is ATI, RadeonHD2600.