Cleaning the rollers will sometimes offer a short term solution. If you are getting a jam as the paper exits one of these type printers this will offer a solution as long as you want to put up with having the paper come out the back. There isn't any temporary solution. If it is jamming as it comes out of the printer and you are getting an accordion type of jam, this is what I'm talking about. It's amazing that they go through some printers at all.
Envelopes vary in size and paper quality.
Watch for unusual printing on envelopes and the same is true for letter head. We have seen some pretty wild and colorful letterhead and a lot of it has trouble going through a printer. The ink provides a slippery surface for the paper roller to deal with. While we're on ink, make sure that the ink on letter head is rated for laser printers. Make sure you remember to tell your printer to use this ink when refilling your letterhead order.