1990 cutlass 3.1 miss

I have a 1990 cutlass supreme with 188,000 miles that miss's at while ideling , it act's as if the coil looses power for a second and then come's back on, when running at highway speed's in overdrive you can notice it also doing it, but not when under load. sense this started happening I have replaced the spark plug's and wire's and repaired a major vaccum leak. in the last 6 month's I have also replace the fuel pressure regulator because of a starting problem, 2 fuel injectors, the alternator and battery , and last fall the coil pack and fuel filter.
Any Ideas of how to fix this ?