If Copy is on Color or BW you will get better results than from Auto Detect
Make sure that Background Suppression is not enabled.
Post a pdf of your job you can print but not scan and I will test it in the lab on the 7970 as well as the 7855, 5955,C60,560,DC260 and the 9303 and see if there is an issue with your machine (I think there is) or an issue with Xerox in general (Tested by Product, product family and other Xerox families)
Specs (pdf link)
Copies are 600*600DPI
Print is up to 1200*2400DPI
I highly suggest updating the firmware, but it won't help the current issue, but the latest firmware (ends with 19700) is simply better than the General Release from shipping. (ends 34540)