| Are you using the empiron or Fiery controller? Might have some to do with calibration if its the fiery. Try the alternate driver to the one you are using, PS vs PCL regardless of controller type. Check the print quality setting. Text/photo vs CAD for example. Check the file itself. Is it a moire patterning? Try reproducing that background colour through tech rep halftone. If it is printing through a fiery, check to see through CWS, via preview image to see if the file being sent to the fiery is not being corrupted at the fiery level. Check the colour profiles.
Check for some of the obvious points like IU bias and transfer bias on the belt and second transfer. Sometimes the units do not seat well and need to be reseated. This is common with the black IU.
Check in tech rep for your transfer settings. If in doubt, zero the values seen related to dmax and background voltage settings and transfer output fine adjustments. Refer to chapter 10.6 for details on these. Reset and stabilize.
Check thick paper transfer values, those might be out as well.
As for thick paper, when you are experiencing washout, that is almost certainly due to the thick paper and you need to set the paper tray according to the weight of the paper being used. Bypass tray has a maximum 300gsm at thick 4. All the other trays are maxed out at 267gsm at thick 3.
Chapter 18 covers some image quality troubleshooting steps based on image reproduction. There is reference there to transfer voltage values, what to check and the like.
Check to make sure your firmware is latest base level.
Failing all that, these machines sometimes will just refuse to cooperate, forcing a necessity to replace all the IUs in one go to correct the IDC variance. The transfer belt itself may be at fault, or even the 2nd transfer voltage bias or the roller itself. | 
Linear Mode
