Unable to connect to iMac via Apple Remote Desktop 3
Try using the "kickstart" command on iMac2: 

sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -configure -users USERNAME -access -on -privs -all -clientopts -setmenuextra -menuextra yes -restart -agent -menu -console

(Obviously change USERNAME to whatever local account you wish to use to control via ARD).

Remote Desktop is still a really finicky, crashy beast (even on 3.6).