Hi,
We use the IQ-501 mostly for calibration. We are not a print shop and use the machine for specific applications, printing always the same stuff on the same 3 types of paper. We only have one thing we print duplex, 485 mm x 320 mm 320 gsm. That paper is cut by our supplier as we don't have a guillotine large enough. The IQ registration process does its job whenever we run it and back to front registration is spot on... for a while until it's not anymore. We do not use the automatic registration (setting up the IQ to check registration as the job is printed) because we mostly run short job (a few 100 sheets at most). The problem mostly comes from our paper and the way it's inaccurately cut... Not much we can do.
One thing I learned is to make sure the tray is set-up correctly, with all the guides secured in the right place. I usually need to make sure whoever fills up the paper knows how to set it up (most of the times they don't!).
Hope this helps.