QTR-Quadtone RIP

Dear fellow photographers,
In one out of 3 prints I make, I have a problem trying to center them, particularly when 
they are oriented for Landscape. It's not just slightly off centered, it prints completely to 
one side, to the edge of the paper. The images are always sized to fit within the paper 
(Tabloid), with enough space for margins as well. Sometimes it works and sometimes it 
doesn't. I have wasted a lot of paper and ink trying to narrow down the problem, but I still 
can't work it out.

My setup is:
Mac OS X latest.
Aperture for organization.
Printing & editing with Photoshop CS2
Printer (new): Epson R1800.
Page Setup: Format for: QuadToneRIP (2.4.3) Epson R1800
Paper: Hahnemuhle Photo Rag Bright White (11x17).
Ink: Neutral K7
Paper Size set up: 'Tabloid'.
Position: 'Center Images' checked.

I cannot figure out why sometimes it prints centered and straight forward, and there are 
times when it prints completely to one side, starting from the edge of the paper. I tried 
using Customized paper size setting (11X17) instead of using 'Tabloid', using both options 
for Landscape orientation, tried sizing the image to fit, in 'Image Size' in Photoshop, and in 
'Scaled Print Size' in the Printer With Preview Panel.

I have tried everything I know how, and still I cannot rectify the problem. Please, can 
someone help? and find a solution for me.
Looking forward to your reply.

Lionel