The paper thickness lever maybe improperly set when printing on an envelope or multiple pages. This causes the metal part of the paper guide to press against the ribbon cloth during printing which 'scrapes' some of the ink off of the ribbon and deposits it on the guide. If enough ink builds up, it will begin to smear onto following pages.
Solution: Remove the paper guide from the carriage assembly using the procedures outlined in the Tech Procedures. The metal shield on this part will probably be coated with a layer of ink (possibly thick). Use isopropyl alcohol to thoroughly remove the ink. Be careful not to bend the metal shield.
Re-install shield as close to the platen as possible. This will put the ribbon a little farther from the paper guide helping to prevent ink from building up. To prevent this problem, adjust for proper paper thickness when printing on envelopes and multiple pieces of paper.
If the above cure doesn't fix the problem, search on GTS and the product's name to obtain a general troubleshooting procedure.