My xerox phaser 6250 prints the first page fine then the black fades on the following pages

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try changing the settings for your printer and see what that does i had the same problem. i changed the settings and it printed normally

You must change Imaging unit.

I am having the same problem. Sometimes my Xerox Phaser 8500 prints a PDF or web page fine, and other times I get pages and pages of what looks like PostScript code. I thought I had installed the most recent printer drivers, We are using primarily Macs with OS 10.4 or 10.5. One recent change is that we connected the printer to an Airport Express using a USB cable, which is sweet when it works correctly....any ideas???

Sounds like it is time to replace the imaging drum. This is a consumable and usually done every 15-20k pages. Cost is about $250-300 and very easily replaced, fast as changing a toner.

I am assuming that you tried changing the yellow toner already? If not, do that first...

What you are describing is a bad print head. You can try running the eliminate light stripes several times, and jet substitution, but most likely the print head will have to be replaced to remedy your problem.
You can try googling. Phaser printer print heads or phaser printheads.
There seems to be a secondary market appearing for those print heads. I can tell you from Xerox, if you have no maintenance agreement, they cost $750 per print head.
Honestly, having worked on about every type of color printer known to man, the new solid ink phaser printers, produce a dry ink image of 2400 x 2400 dpi with ink particles that align end to end. The resolution of those printers are remarkable and can be purchased for less than the price of one of those new print heads.
Now for reliable, albeit somewhat not as sharp a color image, but still exceptional, you really can not beat an HP.
The ink stick technology really no longer has the color vividness advantage it initially had. The inks that align end to end, have ended that advantage and are sure a lot cheaper to operate per page.

I did a combination of the things listed in the below link.

http://office.support.xerox.com/SRVS/CGI-BIN/WEBCGI.cgi/,/?St=180,E=0000000000023462887,K=8165,Sxi=16,Case=obj(19377)

I had this problem when I used a generic toner. I installed a xerox cyan toner, then printed many sheets of a large blue rectangle. After about 15 pages the blue started to spread into the shape, after 30 pages the blue was back. Printing a lot of blue was necessary to get the new toner thru the system.

http://office.support.xerox.com/SRVS/CGI-BIN/WEBCGI.cgi/,/?St=842,E=0000000000019251353,K=6024,Sxi=13,Case=obj(19377)

Click on Knowledge base from this site , then in the search box enter poor black quality. Look under black is faded or missing

That really sounds like the imaging unit cleaner has an issue.
If you have one try the imaging unit, I really suspect that is your problem.
Black is Faded or Missing Step 1 Clean the Laser Windows and Remove Excess Toner From the Imaging Unit Step 2 Check or Replace the Imaging Unit Gear Retaining Pin Step 3 Replace the Imaging Unit (installation instructions)
See if this link works. Xerox is infameous for these not working.
http://office.support.xerox.com/SRVS/CGI-BIN/WEBCGI.cgi/,/?St=842,E=0000000000019251353,K=6024,Sxi=13,Case=obj(19377)