Why does my engine occassionally miss

engines
shifters

#1

I have a '94 Buick Regal with 135,000 miles. At the point where it down shifts to give more power the engine begins to miss and the car chugs. I’ve replaced the spark plugs and wires. It was chugging at 2000 rpms at the point of shifting and the rpms went to 3000. I just had the throttle control replaced and now it chugs at 1500 rpms just before the car down shifts and the rpms go up. Other than this problem at the point of down shifting the car runs perfectly. As the engine gets hot the chugging decreases. Does anyone have any ideas?