I searched this issue in our online knowledgebase and it sounds like you have done all the suggestions.
So at this point I would recommend contacting your local Xerox Support Department to speak with a Product Specialist for more assistance.
I've got a new B600. It printed fine for a few weeks but now everything is splotchy. I've replaced the fuser and drum cartridge with no help. The attached image is an example. Any help with be greatly apperciated.