8500dn paper jam 100% each time

SOURCE: Paper jam message

A couple of things.
On the left side of the paper path, there is a registration flag type sensor that a small piece of paper can be torn off on.
Also, if the paper has been caught on that flag, it may be broken.
Carefully look with a flashlight for just a corner of a piece o paper left in the machine.
I suspect possibly the flag has been broken on the registration sensor.

SOURCE: Paper jams

This is a clasic case of the rubber feed rollers simply being worn and dirty. You can check the clutch to make sure it is turning the feed rollers by barely starting a feed and stopping the machine. The rollers will be in a different position. A second way is to take the paper tray out and trip the paper tray switch all the way in back, then start a feed and put (just a LITTLE pressure) with your finger on the roller to see if is aggresively turns. This will prove that the clutch is good.

DO NOT USE alcohol since this will clean the rubber rollers but make them fail much sooner. Use eucalyputs oil (4 oz bottle in health food stores for $5 will last for years) with a cloth and clean the rubber pickup rollers. If the leading edge of the rollers looks a little shinny you need to barely roughen the entire surface of the roller up with (60 to 80 grit) sandpaper (especially the leading edge) and then clean it again with the euclyptus oil. This will take about 20 minutes to dry and your rollers will pickup paper as good if not better than when they were new.

SOURCE: Paper jam warning icon flashes but there is no paper jam.

Well I would take a can of air and blow off the sensors, especially where you began to have problems. The sensors (over time) become coated with paper dust and now reflect back on themselves thinking there is paper there all them time. From your description of the problem, it seems like the issue-intermitten jams/intermitten false jams, until a total false jam.

SOURCE: I have a xerox phaser 3124 which have a paper jam

Xerox printers have known issues with the rollers. You may want to look into getting some new rollers, they're not that expensive. I think with this model there's one in the tray, and one up above where the paper enters the printer.

SOURCE: Printer says paper jam, but no paper is jammed

Check the following information.......
http://h30434.www3.hp.com/t5/Other-printing-questions/Paper-Jam-Error/m-p/305772#M20513
or
most of the time for a paper jam, there is a QUEUE of papers waiting to be printed. id take out the usb cable via the back of the computer {the one that connects to the printer} go into the control panel, look for printers, double click on that, find the printer, it should say {working offline} right click that, choose the uninstall/delete opts. turn off the printer. reboot the machine, wait until it boots all the way up. take the printer, turn it on, then plug back in the usb cable to the back of the machine. if it is windows either xp/vista it will auto find the usb cable and install automatically. you can then go back to the control panel, printers and it should be listed in the directory. try to right click go to properties of the printer and do a TEST PRINT, should print out.