Did you clean the T/B before you put it on the car. It sounds like it might be sticking. I would try taking it off and give it a good cleaning. 
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Steve Stumbaugh
89 LX 5.0 5sp, Removed air silencer
Mac High Flow Air Filter
Motorcraft 8mm wires
Autolite platinum plugs
Mac O/R h-pipe
Flowmaster American Thunder
3:55 gears
Accel Super Coil (42,000)
75 mm Mass Air Meter
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