2002 Hyundai Accent Misses when accelerating

Has been to the Hyundai garage twice, Goodyear car centers twice. Service engine soon light comes on, Engine misses when accelerating or when RPM's drop ( no tach so I can't say for sure exactly when it starts) Does not seem to happen in first gear. OBD show codes for Cylinder 3 misses. Fuel system cleaned by Goodyear and also replaced number 3 cylinder fuel injector. Problem solved for approx. 3 days. Then Same problem again, misses when accelerating or RPM's drop, Check Engine soon light comes on, as soon as computer re-set runs good for 2 to 3 days then same problem. Both garages say can't find problem? Both service centers report the same code, cylinder 3 misfiring. Both service centers re-set computer, car runs ok for 2 or 3 days and then problem appears again. Hyundai said they even hooked up diagnostic computer and drove it for two days.