My car will randomly jerk while I'm driving it. what could it be? on 1994 Hyundai Excel

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

Asked by for the 1994 Hyundai Excel
It seems that your spark plug is on the side of high gap between spark plug body and contact point area for ignition.
Please change spark plug or maintain the gap between spark plug i.e .003 inch