The issue that you are experiencing may be due to a dirty maintenance kit wiper blade or a dirty printhead. Help can be found in the Xerox Online Support Assistant here on Xerox.com. Here is a link to the Phaser 8550 Knowledge Base solution that will help you clean the blade or printhead. The knowledge base article contains step by step instructions to help resolve the problem.
Hopefully this helps to resolve your problem. If not, at the top of the knowledge base window you will find the 800 number for contacting the Xerox Customer Technical Support (800-835-6100)
I have a 8550DT that sits for days in standby mode. Upon waking - doing a test print I get some uneven (don't know if its considered streaking). Ran a full page single color test as well as a missing jets test. In the single page black test It prints black but it is very streaky consistently down a few areas of the the page almost lines. It prints in black but it seems like yellow is also printing. Did the same for other colors and it almost seems like I'm getting some color corruption. Cyan seems to also have magenta printed out.
Do the run several times and the pages do get much better. With the black its a full page of black, etc.
Maint kit says 50% left - always used Xerox inks. Any clues - no errors. Latest firmware installed.