Is your problem affected by humidity ???
I have exactly the problems listed here about the missing/stuttering in overdrive on my 92 SSE.
Mine also gets worse when the car gets hot or in warmer weather.
When cold, it behaves perfect. On a cold day, it wont start doing this until I have been driving for about 15 minutes. On a hot day, it starts after a few minutes.
The other night, it was warm here (Indiana) but very humid with fog and it didnt stutter at all, even after driving around town for 30 mins.
It seems like moisture or humidity could affect this. Could it be an O2 sensor, EGR valve or PCV valve ?
I've done the usual plugs, wires, fuel pump/filters ect.