That was always a problem with my (almost 20-year-old) Lexmark Optra R. Some types of envelopes seemed to be more susceptible than others, and it was pretty random. I suspect climate had something to do with it as well -- higher humidity days obviously worse than in low humidity winters.

The older enveloper stock might actually be in your favour, IMHO, as the glue will be wettest when the envelops are 'fresh' so to speak. .