What would cause the transmission on a 1992 New Yorker to jump into passing gear without stepping on the accelerator?

It sounds like your torque converter is jumping out of lock up mode. It seems the transmission is reading the pressure incorrectly and thinkingit needs to drop out of torque converter lockup in overdrive and drop down to the next gear. I am not sure if your tranny is regulated by vacuum or by electronics. In the old days they were controlled via vacuum modulators by new ones are electronically driven to match gears to rpms and ground speed.