What could be causeing jerking when changing gears in an 2001 grand am Pontiac?
Assuming this is a 5 speed car, the most common reason for this is worn out CV joints. Could also be a bad torque strut.
Why has your Opel Corsa started jerking as if you are tapping the accelerator when your foot is steady?
It could be several things but if it is a constant jerking (repeating) and more noticeable a lower speeds < maybe even pulling to one side ,where you have to hold the wheel a little to the other direction to go straight. I would suspect a bad tire , may not be noticeable just looking at the tire but could have a internal broken belt.