It seems likely that high humidity levels may be accelerating the chemical process that is causing the ink to bleed and offset onto the previous page. If there is room in the comb bindings, it might not be a bad idea to interleave the pages with a material like a lightweight interleaving paper or glassine. These could then be changed out periodically if needed, and at least the offsetting wouldn’t disfigure the previous page, potentially making legibility more difficult. If there are any measures you can take to control the environment, this should be done ASAP. Even installing an inexpensive digital RH and temp reader would be better than nothing. You could then collect the data on a daily basis and use this to help formulate a plan to control the humidity by installing a dehumidifier if high humidity is indeed a culprit.
It may also be that the toner used when they were copied is chemically unstable in the first place. I have worked on some documents from this time period, and they tend to have inconsistent inherent condition issues. I am sure this is only the beginning of the problems we paper conservators will face in the coming decades when dealing with these materials.
One other thought – perhaps it would be a good idea to go ahead and make new copies (both digital and print), so you will be able to retain as much existing information as possible. Printer inks have come a long way and I would recommend you print on a paper such as PermaLife to ensure maximum stability of the paper.
Hope that helps!