I rather doubt Lr is the primary cause, because the choice of Black depends mainly on the Media Type set in the driver. It sounds to me as if there is some autonomous change of settings happening with the printer driver causing this. Are you printing using profiles or "Printer Manages Color"? You will need to do some diagnosis. Let this continue happening for a few cycles regardless of the annoyance and the cost. When it does happen, go into the driver settings and see whether the Media Type has changed from where you had set it to something else, or a Printer Preset has changed from where you want it to something else. Check in the printer controls, usually on the printer LCD, if the option is available whether PK/MK change is set to automatic or manual. If it is set to automatic, switch it to manual. I don't know the specifics of that driver version any longer (it's been years since I gave away my 3800) so I can only suggest to poke through the various setting options to make sure nothing is set that would trigger autonomous changes between Mk/PK ink. In the final analysis if you can't trace the cause, call Epson ProGraphics (this is a qualified Pro printer for ProGraphics support) which is free over the phone, and seek their advice on whether there is any incompatibility they know of between the driver version you are using and the computing environment it sits in.
