There were massive changes in both the film (C-22 Kodacolor-X going to C-41 Kodacolor II and later Ektar) and paper (the original Type C going to Ektacolor) during this period. There were actually many significant changes to the materials and the processes, not all of them well publicized. As a result, results vary. Some materials and processes were better than others, and sometimes labs didn't get the word on how to handle things, or just ignored the new instructions. There were also a number of non-Kodak proprietary processing solutions that might or might not produce stable results. I have a lot of material from this period, both C-22 and C-41. Most of it is still in good shape, but some has faded too badly to use. Anything shot of Vericolor II is gone. My guess is that this film, beautiful as it was for its time, was just not stable. Many things processed with non-Kodak materials are gone or going.