Printing Problems (Intermittent)

Desktop OS = Windows XP Pro
Application = Office (word) XP Pro
Printer = HP LJ 4050n (Networked)
Printer RAM = 64Mb
Printer Drivers = Downloaded from HP
Printer has a duplex printing attachment.

When printing sometimes the bottom section of a page or all pages has not printed.

There is no problem when using a Windows 95 / Office 97 combination on this printer.

User description:
I just printed 15 letters in a mail merge , back to back (duplex), and the bottom third of the firstpage didn?t print?on any of them.

I had previously printed 2 batches of letters using the same mail merge file, one of 56 and one of 11, all of which fortunately did print in full.
The problem batch of 15 was printed within minutes of the first 2 batches.

Any help would be appreciated