So I've had my car for a few months and put about 2k miles on it. I recently got an amp and subwoofer installed. I drove about 40 miles then noticed when I stopped at a redlight my rpm gauge would go to about 500 and stay around that area. When you release the brake it stalls a little bit then drives fine. Usually, it stays around 1k rpm. Im not sure if its just me being worried but it also seems like it is shifting a little different then normal. Anyone have any ideas?