2001 Hyundai Elantra Suddenly Missing and lost power

I had the exact same problem with mine and it near drove me crazy. I changed the fuel filter, spark plugs, air cleaner the works and still could nt solve it. One day I lifted the spark plug lead off the plug as the engine was running and noticed cross firing from the rubber boot to the cylinder head when the boot was just above the spark plug. I lifted the plug wire with a piece of wood to avoid getting shocked. I went and spent $55 on a new set of leads and it was fixed in an instant. Good as new.
I am not saying this is your problem but one you might investigate. A small hole or crack in the plug boot is all that it needs. I could nt even see a hole or crack in mine but I could see where it was coming from by the spark shooting out and on top the head.
Good luck

I traced mine back to the coil. Unplugged the number 4 cylinder and noticed that there was no change in the way it ran. Took the wire and placed it in number two. No change. Unhooked it and it wouldn't even run. So I know the wire was good. Put it back in its original place and it went back to missing on number four. One side of the coil was apparently burnt out. Replaced and boom. Runs great.