Okay, I recreated the document as letter sized with the necessary margins. I did discover I had to make it landscape for it to work with the way my printer takes envelopes. I found that stapling the envelope to letter sized paper and printing it caused a paper jam, so I'm printing just the envelopes by themselves.
It worked about 3 times, then it started shifting off the envelope again.
So I tried adding the amount that it got cut off to the margins and it worked again, then started shifting.
Then it was haphazard. It would print fine once or twice, then it would shift for the next 3 copies.
I'm printing them one at a time, because I found printing more than that at one time caused paper jams.
I started thinking maybe the guide in the printer tray that holds the envelope in place was getting shaken out of place by the movement of the mechanical parts in the printer, so I started re-setting it every time to be sure it was snug. But again, if started shifting after a couple of prints.
Then I thought, maybe the envelope's flap was getting pulled back somehow inside the printer, so I tried tucking the flap inside the envelope, but it made no difference.
So far I've got only 19 good copies and 24 ruined envelopes.
I really don't know why this isn't working or what else to try? Any ideas?
Maybe it has something to do with friction building up? Maybe I need to take a 5 minute break between prints? Still, I have to print 50 of these! That's going to take forever!