Are you using the systray client because something seems off about the 'Target:' line.So have 2 9800GX2's in one machine and I am having a rather hard time getting the 4th client to work. The first three clients are working great but the fourth gets an 'UNSTABLE_MACHINE' before it even tries.
In the shortcut I made for the client I have the target:
And Start in:
Everything looks right to me.
On my systray machine both shortcut target lines are the same except for the -gpu X at the end.
You might also try adding the -local flag if you have not already...