2006 Pontiac Solstice Car clunks when slowing down

I have a 2006 Pontiac Solstice, with about 34000 miles. It is an automatic transmission. As I start to slow down the car will jerk at 3 different intervals, each at a lower speed. Slowing down to around 30mph will produce the first medium size jerk. Then at 20mph will be the next jerk this one being the strongest, and at around 10 mph will be the final smaller jerk. This process happens each time I'm slowing down at a light or stop or need to slow down in general.

However, lately when I know I need to slow down, such as at a red light, if I put the car into neutral, I can slow down at each of the above points, nice and smooth. No problems whatsoever.

Because of that, I'm not sure if it is a mechanical problem or an electric problem, or maybe a little of both. I'm hoping you'll be able to help me out as I'm really wanting this problem to stop as it is very annoying, especially in stop and go traffic.

Thank you.