I have an iMac and an LG Flatron connected as a secondary monitor.
The recommended resolution for the flatron is 1680x1050 @ 65.290 Hz (horiz), 59.954 Hz (vert).
For some reason, OSX is choosing a slightly different set of scan rates and this is currently my best guess of why the monitor goes into power saving mode when connected to the iMac (but works fine on a PC).
Now, I resolved this by installing switchResX and fudging the scan rates according to the specs in the manual.
But how does one change these rates w/o 3rd party tools? Which config files need editing?