no matter how long i have the car running or the temp of the engine or outside air temp, I have a slight sputter right around 1200-1500 rpm. Once it passes that, it is fine and runs smooth, even under heavy acceleration, and below that, it runs as smooth as can be, but once it hits that rpm range, it feels like it misses a few beats. Anyone have any ideas?

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1996 E300 D
1992 300D
1990 Ford E-350 7.3 Diesel AKA "the Deep Fryer"