Mark,
Something worth checking:
PS always tries to center the image on the printable area, not on the sheet of paper. That would not matter when you print on roll media but would produce off-centered prints on cut sheets as the minimum bottom margin is way bigger than the top margin.
The best workaround on HP Z3xxx printer is to configure your printer's settings this way (my driver is not in English, so the translation may not be perfect):
1) Avoid auto-selected paper source, always select roll media or manual feed accordingly.
2) On "Margin/border Layout" select: "Print with margins/borders" and "Cut contents by margin" (This is different of borderless printing as it will report borderless to PS but will not actually print on the minimum border, if there is image over that area it will go blank).
That is usually enough to solve the margin problem, but you may also go ahead and size the whole page before printing. Set your image size as you want and grow white borders around it using "canvas size" to crate an exact replica of the sheet you will use and print with the "cut contents by margin" selected. Remember the unprintable area of 1,7mm on the trailing edge and 5mm on the rest.
I don't know if that is the source of your problems, but it surely was a headache to me when I first got this printer.
