1992 Toyota Camry car jerks while idling

While at a long stop light and my foot is on the brake I would feel my car jerk a little and noticed the RPM would dip down and go back to where it originally was.
Later in the day, the jerks would occur more often once again at stop lights and then sometimes when I start the car and the car is idling.
I've had a tuneup, transmission fluid changed, clean intake induction system, clean throttle body, and fuel injection system done.
What do you think is the cause?

Thanks,
Susan

PS: Will be driving this car across country had dist cap, rotor & set timing, checked hoses, belts, fluids, battery, tires, & brakes checked. Anything else should be done?