2002 Grand Prix GT - Jerking issues

I have a 2002 Grand Prix GT with roughly 135,000 km's on it. The problem I'm having is that I am getting a jerking motion in the transmission while the car is in drive. It doesn't matter if I'm doing 10 Km/hr or 100 km/hr. The GM dealership can't seem to figure it out. The jerking is very noticeable. It feels like the car slips out of gear for a moment, then goes back into the same gear again . Can anyone help me with how to approach the issue with the shop?

Now the check engine light is on again (which was reset previously).

The car has been perfect up until this point. Any suggestions would be greatly appreciated!

Welcome to AF.

What was the diagnostic trouble code(s). Get the DTCs before resetting and post.