Plot preview default to mm

I posted this problem a year or two ago and at that time no one else seems to have had the following problem (maybe by now someone else has):When preparing to plot, the print settings dialog sometimes changes settings by itself from "inches" to "mm" and in so doing, the drawing shows up as a tiny object on my 24" x 36" sheet in the preview window. When I set the dialog back to "inches" all my line weights change to "very " thin lines corresponding to the "mm" setting. But when I preview again, the "inches" changes back to "mm". The program doesn't hold my settings. Is there a fix for this or a different print process I can use for my HP 430 plotter?