I have a 2004 Pontaic Grand prix gt ......I will be driving along come to a stop light and as I am begining to move the car will start shifting so hard feels like the trans is falling out once past 1st gear (this is an automatic)smooth sailing . Can pull over to the side of the road turn the car off restart and it stops for days, weeks, what have you and then it will do it again, I have had it to the GM dealer and private mechanic , no codes, and cannot get it to happen when they have the car. I have heard that there is a code P1811 that will not make my car check engine light come on also believe that this has something to do with a sensor or solenoid in my car. Has anyone else had this problem and if so what was the outcome. My car only has 84000 on it has been taken care of not ran hard and always maintained to the owners manual of oil, fluid, changes, etc.......

Thank you in advance for any solutions
Janis