1991 Buick Century Custom 6 Cyl 3.30L
Rough idle. Almost stalls.
As car idles after warm idle drops and then jumps higher like about to stall then goes higher. Up and down.
Anytime a GM car has more rough running after warming up and the year is between '87 and '93 the first suspicion is shorted fuel injectors. Just Google GM multec fuel injectors and the symptoms which can vary a lot, but the red flag is that the car usually runs seemingly normal until the temp gauge starts to reach operating range and then the erratic running begins. There is a way to check them. Do you have an digital ohmmeter?