My fuel filter light flickers when my truck is idling. When I move off idle the light goes off. I would think if the filter is plugged this would be the other way. By the way the filter is not that old.
Engine vibrations change from idle to higher RPM. You might also check the vacuum switch. Just unhook it and see if the light still flickers. It obviously isn't a fuel filter stopped up, because it only flickers at idle only.