Well you might try looking for updated drivers if you havent already. If you have the most current you might try switching between PCL or PS drivers on the laserjet. I dont rem deskjets(your 960) having that option.
If that doesnt help you cantry updating the firmware on the printers/jetdirect if possible. You can use Web Jetadmin from HP to do this and works real nice. I rem having a quirky issue with some highend HP lasers that wouldnt print WEB docs/images/etc until we updated the firmware on the printers/JetAdmin cards.
Also try adjusting settings for the printer. When you goto printer properties there should be an advanced tab with a button that says print processor.(may vary from different printers. going by a 8000 series setup) Click there and try adjusting from RAW to the other settings. However I have usually seen better results under RAW. Try other properties to see if it makes it better.
Depending on the size of the documents it could be a memory issue. Make sure there is plenty of RAM both on PC and printer. XP works best with a minimum of 256MB. Usually lack of memory will just slow things down though.
This is all I can think of for now.
Hope this helps
Jason