I had the same problem with an '89 Toyota Corolla twice. The first
time, it was bad spark plug wires. The second time, it was a broken
"throttle position sensor." I have no idea if a Buick Century has
this sensor too.
The cheap route to take may to be to change the spark plugs and spark
plug wires first. If that doesn't work, see if there are any computer
error codes that you can pull from the car's computer (I don't know
the procedure) that may point to if a sensor is bad.
Hope this helps,
'89 Toyota Corolla 129k
'87 Chevy Celebrity wagon 113k (same 2.8 as your car, but I don't
recall ours having had this kind of problem before)
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