My 2000 Buick Century has 60,000 miles. When driving long distances or at higher altitudes it shifts unexpectedly and with severe jerks - both down and up-shifting. Speed and incline often aren’t obviously enough to warrant the shifts. Extended warranty expires in ten days. Since I live at sea level, how do I replicate the problem for a local mechanic? And what might be causing this?
The pressure control solenoid in the transmission may be worn.
This solenoid used to be a culprit of shifting difficulties in the 2000 Buick Century. (At least it was in two posts I read)
Another one to check out could be the VSS (Vehicle Speed Sensor).
Perhaps the computer needs to be reprogrammed. Just guesses mind you.
Get it scanned… One of the inputs that the computer uses in the shift strategy comes from the MAP sensor. Let a trans shop or dealer have a look. It could very well be a driveability issue and not a transmission issue.