imac doesn't play friendly with lcd projector

I have an imac g5. I can't connect through the video port to the lcd projector. This worked up until a couple of weeks ago. I can connect the lcd projector through the same adapter/cable to my macbook with no issues. When I go to system preferences it only shows the imac display and the lcd projector continually searches for a signal source. I have checked to be sure i have the same resolution settings on both the imac and the lcd (which is a dell by the way.) What else can I try?