Ian,
What model Image Pro are you using...SDI?....HD?
Are you seeing this issue when you hook up a monitor to the passthough port on the Image Pro? or only at the projector?
It sounds to me from your description that it might be either a Gamma adjustment issue or the polarizers in your projectors.....odd that it goes away at a different resolution though.
Have you tried swapping out your Image Pro for another to rule it out?