Propulsion system problems

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After stopping at red light or stop sign when I take off my car will stop suddenly and then jerk forward and go again. This seems to be happening more often. I am very worried thst I am going to get rear-ended. I took it to GMC and they said the motor mount was broken. That has been fixed I have had car back for 2 days and it is happening again. Sometime the check engine light will come on after this happens, and On-star said the code was for engine propulsion system. Any ideas what is going on. I have seen others complain about this but jave never seen a ansewr to tje problem. Please help! Thank you.