hang on, what? load a monitor?
all monitors that are in devs:monitors get setup on startup. then the machine will be able to understand the screenmode.prefs file and use the screenmode you saved.
if it isn't giving you your "saved" screen mode, then it sounds like you are either missing the monitor in devs:monitors (drag and drop from storage/monitors), or missing the initialization part of your startup sequece?
whats your setup? AGA? P96? CGX?