97 Buick Lesabre 3.8 120k

The veh started missing about a month ago. I changed the plugs & wires. Veh still misses. Misses at start up & when warm. When in park, you can rev the engine with no misses till you get to 4000 rpm, then a miss will occur. When you bring the veh back down to idle, it will idle smoothly for about 15 to 20 seconds, then start to miss. When you take off very lightly, the veh will not miss most of the time. When you take off normally, the veh will miss. Once you reach cruising speed, 25 to 70, city or highway, the engine does not miss. If you try to pass on the freeway it will then miss. Trying to accelerate thru an amber light causes a miss. Engine light has been on for 5 years, always ran fine till amonth ago. Any idea what it might be that I can fix? No other service done other than oil changes & brakes, air filter, tires.