Dell 3100cn printer prints badly on envelopes and labels

I have a Dell 3100cn color printer which works fine on plain paper.

However, when I try to print envelopes (which I put through the mpf)
it prints the address at the right location and then a faint but
definite image of the address a few inches to the left of the real
address. This makes the envelope unusable.

Second, I have tried to print Avery 6267 return address labels with
fairly disastrous results. I prepared the table of labels which work
OK on plain paper. However, on the Avery labels, many faint copies of
my address are also printed and overlap the real addresses.

It doesn't seem to matter whether the paper comes from the lower tray
or from the mpf.

Any suggestions?

Thanks,
Bill Edelstein
Baltimore