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1988 car jerks

1988 Mercury Grand Marquis

It jerks on occasion. Seems to do this at the point when it should downshift, but does not downshift. If I press down on the gas so that it downshifts then it runs fine.

Does it worst in cruise control. If I take it off of cruise control I can ‘feel’ when it is about to jerk and I can prevent it from jerking by either letting off on the gas a little bit, or pushing down on the gas so that it downshifts.

Some of the time in cruise control it downshifts very smoothly.

It was getting worse and some good injector cleaner added to the gas seemed to help but it never did go away completely.

Lately it has been getting a lot worse, mainly on moderate upgrades. Have not had a chance to get more good injector cleaner recently.

The jerk is almost like the computer tells the engine to shut but then changes its mind.

1988 Ford Crown Victoria

Did a similar thing, mostly on fairly flat ground at around 35 MPH constant speed, when it seemed to be indecisive on whether to downshift or not.

New plugs, air filter and gas filter are typical firsts, any maintenance history you could add?