I purchased a d80 recently

Question

I purchased a d80 recently and took about 2,000 shots at an outdoor music festivalvirtually 90% of them came out with poor image quality, i was shooting in "fine" mode and alternated my settings primarily from portrait, auto, P, settings- example i was taking a picture of a trombone player wearing a white shirt and black pants, when you look at the black, it looks like scatter multicolored pixels, like an image taken with a 2 megapixel camera from 500ft away and then enlarged. I was taking the shot about 10ft away, essentially all of the photo had this multicolored pixel look. 18-55 kit nikon, 70-300vr nikon lenses. I've taken other photos and not had this happen.Help!