1990 Mercury Grand Marquis Poor normal acceloration over 30.

This car is killing me its got a complete tuneup in it fuel filter. Its alittle better still it kicks into what feels like 4th gear while in overdrive and it falls on its face, but when I put the peddle to the floor and get passing gear it goes and seems ok. The car only does this when it is in 4th just rolling down the street normally. Please tell me is it injector problem or throtle body problem. HELP!