2001 Hyundai Elantra Engine misfire and recurring ODB code

My daughter's Hyundai Elantra started showing a Check Engine light about a month ago. I checked the code (0303) which indicated a misfire on #3 cylinder. I changed plugs and plug wires, and reset the code. The car ran for about an hour - several short trips - and then the Check Engine light came back on with the same code. I rechecked the plug and plug wire to #3 cylinder and reset the system. Again, no code for about an hour or 50 miles but then it's reappeared.

I discounted that it could be the ignition module since there isn't a code for the other cylinder fired by that coil. Should the coil module be checked?

The car runs well - no noticable misfire, and it would be pretty evident with a 4-banger. There is a bit of vibration at idle but not a miss and there's no stumble when you push the throttle.