[CentOS] Printing Postscript - Very Weird Problem.

Hey folks,

okay, here's the situation - I have a Centos 4.2 system with cups printing 
installed printing via postcript (with appropriate PPD file) to a HP 
LaserJet 1320N printer (via jetdirect socket 9100).

Everything seems to work right - except for an unusual 'font'??? problem.
Many letters seem to be just slightly garbled.  At first I thought this 
was a mechanical problem at the printer (for weeks), but just this morning 
I took a closer look and realized everything was too regular for it to be 
a mechanical failure (ink smearing or something).

Anyway the problem is that the bottom non-empty scanline of _very_ many 
letters/digits/symbols gets transposed over ontop of the topmost non-empty 
scanline of the same character.

This results in the following conversion (rough ascii art - note there's a 
lot more scanlines involved due to far higher resolution, which means this 
is far more invisible).

|      AA      |    | AA        AA |
|     AAAA     |    |     AAAA     |
|    AA  AA    | => |    AA  AA    |
|   AAAAAAAA   |    |   AAAAAAAA   |
|  AA      AA  |    |  AA      AA  |
| AA        AA |    | AA        AA |

Any idea how to fix this?  What could be the problem?  How to figure out 
what could be causing it?  It's a very good printer which has no other 
faults, and it's printing the above characters very crisply... (not that 
it matter seeing as they are bad...).

Cheers,
MaZe.