New mac mini output too dim

We just purchased a new mini to replace our old ones used to run video in a video sphere. It is connected to an Eiki projector via a dvi connector. Of the 4 minis we have, 2 of them (the oldest and the newest) are considerably dimmer than the other two. I have not changed any settings in the projector. This is getting very frustrating since I need to use the fastest (newest) computer in the sphere. Does anyone have any ideas?