My car will randomly jerk while I'm driving it. what could it be?

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My car will be working fine and then all of a sudden it seems like it "slips" out of gear and will start deccelerating even though my foot is on the gas. Then it will seem to catch and jerk as its catching. Sometimes it will happen while switching gears, sometimes it will happen while I'm in gear. It's very random. It will do it at high speeds, low speeds, average speeds, warmed up or cold, in Economic or Normal gas input. We have put in new transmission fluid, new oil, changed the fuel filter, changed the one of the fuel caps, had the transmission checked, and some other stuff I can't remember. None of it fixed the problem