My buick starts chugging after it shifts into overdrive,then it smooths out. on 1995 Buick Regal

Car starts up,but when you start driving it,and it shifts into overdrive,it starts jerking,or chugging.It smooths out after it gets to a higher speed.

Asked by for the 1995 Buick Regal
If it is a jerking action most likely engine misfire, have it scaned for codes. Most likely a plug wire but scanner can pinpoint which cylinder so problem can be found more easily.
Now if it is a fast shutter or like a vibration, may be a torque converter problem.
A few months ago,i took it to a garage.He said the vacum line was loose.I can't rember if he adjusted it,or replaced it.That stoped the chugging for about two months.Then started doing the same thing.