accelerates to 4000rpm when braking from high speeds

This was not the first time but it is the first time it involved another vehicle. The unwanted accleration usually only happens on the highway, when braking from high speeds (because of rushhour traffic). I have started to drive with my hand on the shifter and shift it into neutral EVERY time I brake. While sitting in traffic today, it suddenly accelerated. I braked really hard, but didn't have time to get my hand to the shifter to put it in neutral. I was able to turn a little to the right, even though I was in the middle lane and there were stopped cars on both sides of me. I hit the guy in front of me pretty hard. He got out and swore at me, told me to look where I was going, checked his bumper for damage. Then he got in his car and drove off. I didn't have any damage; this time it accelerated from a full stop. It scares me to death (nearly) everytime it does it.