Shifting Problems

I have a 2001 Sunfire, when I run the car during the day and I stop and go at lights, the Trac OFF light turns on and the car starts having problems shifting. It shifts really hard from one gear to another. It's an automatic. When I pull over and put it in park, I turn it off and then back on, it works fine again for a while longer. When I pull over and go from Drive to R the transmission shifts very hard.

Is this a sensor problem? I know it's not the transmission it self, when it's cool outside or it's on the freeway it drives fine with no noise or complications, it's just when the Trac Off light comes on.

Some people told me it could be a speed sensor or something with the ABS, does anyone have an idea????

Thank you