Designjet 500 woes

[dazzer21]

I have an iMac C2D running OSX 10.6.8. Connected to this I have an HP Designjet 500 24" inkjet printer connected via USB. It prints fine, except EVERY SINGLE TIME it refuses to print at the correct size. For example, I have set up a document at A1 size (594mm x 840mm) in InDesign CS5. On the page I have placed a keyline box 1mm inside the document (measuring 592mm x 838mm. I have also placed another 5mm in (584mm x 830mm). Via the page setup, it's set to print to A1 with no border. A PDF of the file is created from InDesign with no crop marks and no bleed. Printing is via Acrobat Pro 9.5.5. Again, Page Setup is set to A1 - 59.40cm x 84.00cm. In the print box, the preview shows a page size of 59.373cm x 83.996cm. As a measure, I have selected "shrink to printable area" which does not affect the size of the print - shows as being printed at 100%. When I print, the outer of the two keylines (positioned 1mm inside the print area) does not show at all so for some reason, the print area IS less than A1. The inner keyline (situated 5mm inside the print area) shows either side but is chopped off at the top and the bottom so it's printing at least 10mm short. So - if you have the same sort of setup as me, presuming that it's some sort of driver/software issue, what do you use to get proper spec prints. Hopefully, all makes sense. Well, it does to me, anyway... Thanks.