shades of grey - not that kind

I've had my Mac Pro a few years and am running OS X 10.6.8. Lately, I haven't been able to view screens of colors under 16%. I purchased a new monitor and calibrated it. It still does not show screens of colors under 16%. As an art director, it's imperative to be able to view screens correctly. Does anyone know why this would happen?

Thank you.

Without any details about your system, I can only guess.... Possibly your video card in the Pro is failing. Do you have another you can swap it out with?

Response to video card

I'm pretty sure we have another computer I can pull from, I will give that a try. I appreciate the suggestion, I couldn't find anything online. Thank you!